Translocated c-myc oncogene of Burkitt lymphoma is transcribed in plasma cells and repressed in lymphoblastoid cells

Structural insights into mis-regulation of protein kinase A in human tumors

The extensively studied cAMP-dependent protein kinase A (PKA) is involved in the regulation of critical cell processes, including metabolism, gene expression, and cell proliferation; consequentially, mis-regulation of PKA signaling is implicated in tumorigenesis. Recent genomic studies have identified recurrent mutations in the catalytic subunit of PKA in tumors associated with Cushing's syndrome, a kidney disorder leading to excessive cortisol production, and also in tumors associated with fibrolamellar hepatocellular carcinoma (FL-HCC), a rare liver cancer. Expression of a L205R point mutant and a DnaJ-PKA fusion protein were found to be linked to Cushing's syndrome and FL-HCC, respectively. Here we reveal contrasting mechanisms for increased PKA signaling at the molecular level through structural determination and biochemical characterization of the aberrant enzymes. In the Cushing's syndrome disorder, we find that the L205R mutation abolishes regulatory-subunit binding, leading to constitutive, cAMP-independent signaling. In FL-HCC, the DnaJ-PKA chimera remains under regulatory subunit control; however, its overexpression from the DnaJ promoter leads to enhanced cAMP-dependent signaling. Our findings provide a structural understanding of the two distinct disease mechanisms and they offer a basis for designing effective drugs for their treatment.

A subtle t(3;8) results in plausible juxtaposition of MYC and BCL6 in a child with Burkitt lymphoma/leukemia and ataxia-telangiectasia

Translocations involving 3q27 that affect the BCL6 gene are common and specific chromosomal abnormalities in B-cell precursor non-Hodgkin lymphoma (mainly diffuse large-cell and follicular lymphoma), but they have not been reported in Burkitt lymphoma. Here, we describe a case in which a BCL6 rearrangement and additional complex cytogenetic abnormalities occurred in a child with Burkitt lymphoma/leukemia and ataxia-telangiectasia. Although cytogenetic analysis of the bone marrow revealed clonal abnormalities of chromosome arms 8q and 14p and other subclonal abnormalities, the t(8;14) or its variants typically associated with Burkitt lymphoma were not observed. Fluorescence in situ hybridization with locus-specific probes and multicolor spectral karyotyping demonstrated a complex pattern of chromosomal rearrangements leading to a subtle t(3;8)(q27;q24.1) that rearranged BCL6 and placed it adjacent to MYC. We speculate that this genetic lesion occurred as a result of chromosomal instability due to the underlying disease.

Detection and analysis of spliced chimeric mRNAs in sequence databanks

We have developed a databank screening procedure, the In Silico Trans-splicing Retrieval System (ISTReS), to identify heterologous, spliced mRNAs with potential origin from chromosomal translocations, mRNA trans-splicing and multi-locus transcription. A parsing algorithm to screen cDNA versus genome Blast outputs was implemented. Key filtering criteria were Blast scores of > or =300, match lengths of > or =95% of the query sequences, junction of the two partners at exon-exon borders and concordant 'sense/sense' reading orientation. ISTReS was validated by the successful identification of bona fide chromosomal translocation-derived fusion transcripts in the HGI and RefSeq databanks. The performance of ISTReS was verified against recently identified chimeric antisense transcripts, where it revealed essentially no independent proof of antisense transcription and absence of exon-exon borders at the chimeric join, consistent with an artefactual origin. Analysis of the UNIGENE database revealed 21 742 chimeric sequences overall that correspond to approximately 1% of the database transcripts. Novel FOP-Rho GAP and methionyl tRNA synthetase-advillin chimeric mRNAs with the canonical features of heterologous-genes spliced-transcripts were identified among 246 chimeras from the RefSeq databank. This suggests a frequency of canonically-spliced chimeras of approximately 1% of all the hybrid sequences in current databanks. These findings demonstrate the efficiency of ISTReS and the overall feasibility of sequence/structure-based strategies to search for chimeric mRNAs candidate to derive from the splicing of heterologous transcripts.

Insulin-like growth factor I is a dual effector of multiple myeloma cell growth

Multiple myeloma (MM) is an invariably fatal disease that accounts for approximately 1% to 2% of all human cancers. Surprisingly little is known about the cellular pathways contributing to growth of these tumors. Although the cytokine interleukin-6 has been suggested to be the major stimulus for myeloma cell growth, the role of a second potential growth factor, insulin-like growth factor I (IGF-I), has been less clearly defined. The IGF-I signaling cascade in 8 MM cell lines was examined. In 7 of these, the IGF-I receptor (IGF-IR) was expressed and autophosphorylated in response to ligand. Downstream of IGF-IR, insulin receptor substrate 1 was phosphorylated, leading to the activation of phosphatidylinositol-3′-kinase (PI-3K). PI-3K, in turn, regulated 2 distinct pathways. The first included Akt and Bad, leading to an inhibition of apoptosis; the second included the mitogen-activated protein kinase (MAPK), resulting in proliferation. Biologic relevance of this pathway was demonstrated because in vitro IGF-I induced both an antiapoptotic and a proliferative effect. Importantly, in vivo administration of IGF-I in SCID mice inoculated with the OPM-2 line led to approximately twice the growth rate of tumor cells as in controls. These results suggest that IGF-I activates at least 2 pathways effecting myeloma cell growth and contributes significantly to expansion of these cells in vivo.

Insulin-like growth factor I is a dual effector of multiple myeloma cell growth

Multiple myeloma (MM) is an invariably fatal disease that accounts for approximately 1% to 2% of all human cancers. Surprisingly little is known about the cellular pathways contributing to growth of these tumors. Although the cytokine interleukin-6 has been suggested to be the major stimulus for myeloma cell growth, the role of a second potential growth factor, insulin-like growth factor I (IGF-I), has been less clearly defined. The IGF-I signaling cascade in 8 MM cell lines was examined. In 7 of these, the IGF-I receptor (IGF-IR) was expressed and autophosphorylated in response to ligand. Downstream of IGF-IR, insulin receptor substrate 1 was phosphorylated, leading to the activation of phosphatidylinositol-3′-kinase (PI-3K). PI-3K, in turn, regulated 2 distinct pathways. The first included Akt and Bad, leading to an inhibition of apoptosis; the second included the mitogen-activated protein kinase (MAPK), resulting in proliferation. Biologic relevance of this pathway was demonstrated because in vitro IGF-I induced both an antiapoptotic and a proliferative effect. Importantly, in vivo administration of IGF-I in SCID mice inoculated with the OPM-2 line led to approximately twice the growth rate of tumor cells as in controls. These results suggest that IGF-I activates at least 2 pathways effecting myeloma cell growth and contributes significantly to expansion of these cells in vivo.

DNA, RNA and immunohistochemical characterization of the HER-2/neu oncogene in transitional cell carcinoma of the bladder

HER-2/neu overexpression appears to play a role in determining the malignant potential of some human cancers. To date, no urothelial malignancies appear to have been evaluated for HER-2/neu DNA amplification, mRNA expression and protein overproduction. By Southern hybridization we detected DNA amplification and a possible structural rearrangement of the HER-2/neu oncogene in one of 12 bladder tumors. A 14 kb DNA fragment in addition to the expected 12.5 Kb fragment was found. Additionally, the HER-2/neu oncogene was amplified sixfold in the tumor compared to placental DNA. Five of 14 (36%) bladder tumors overexpressed HER-2/neu mRNA three to 38-fold compared to normal urothelium. HER-2/neu overexpression occurred in superficial and invasive tumors. Immunohistochemical analysis was performed on the one tumor with DNA amplification and the 14 tumors evaluated for mRNA expression. The tumor with DNA amplification and three of the five tumors with HER-2/neu mRNA overexpression stained positively for the p185HER-2/neu protein. These findings suggest that DNA amplification occurs infrequently in bladder cancer. Thirty-six percent of bladder cancers overexpress HER-2/neu mRNA. Immunohistochemical analysis with a p185HER-2/neu polyclonal antibody, on formalin fixed, paraffin embedded tissue, was specific for HER-2/neu overexpression but not as sensitive as Northern analysis.

Elevation of glucose transporter, c-myc, and transin RNA levels by Ha-rasT24 is independent of its effect on the cell cycle

Elevation of the steady-state mRNA levels of glucose transporter and c-myc are among the earliest changes in gene expression observed after Ha-rasT24 stimulation of Rat-1 fibroblasts to enter the cell cycle. Since the expression of these genes may be the result of either increased cell proliferation or a specific response to rasT24, we evaluated the expression of glucose transporter and c-myc and their induction during the cell cycle in both parental Rat-1 cells and cell lines bearing a metallothionein rasT24 fusion gene (MTrasT24). We showed that, although levels of glucose transporter and c-myc mRNAs in Rat-1 cells underwent a transient increase within hours of the addition of serum, epidermal growth factor, or 12-O-tetradecanoylphorbol-13-acetate to quiescent (G0) cells, the levels of glucose transporter and c-myc mRNA otherwise remained constant throughout the normal cell cycle. In cells carrying MTrasT24 (MR5 cells), induction of rasT24 expression by ZnSO4 led to a rapid induction of glucose transporter and c-myc mRNA expression in both quiescent (density-arrested) and G1/S-synchronized (aphidicolin-blocked) cells. These increases exceeded the constitutive levels expressed in rapidly proliferating Rat-1 cells, indicating that the ras oncogene has an effect on these genes that is independent of growth status. In addition, the transin gene, which is not expressed in proliferating Rat-1 cells in the continuous presence of serum growth factors, was also induced after increased expression of the mutant ras gene. These results suggest that the induction of glucose transporter, c-myc, and transin is the direct result of rasT24-mediated alterations in cellular gene expression and is distinct from normal cell-cycle events.

The block to transcription elongation is promoter dependent in normal and Burkitt's lymphoma c-myc alleles

Aberrant c-myc expression patterns occur in human Burkitt's lymphoma cells, which consistently exhibit c-myc chromosomal translocations, mutations within and flanking the translocated allele, a loss of the block to transcription elongation in exon 1, and a promoter shift to use of the upstream P1 promoter. To define the mechanism responsible for the loss of transcription elongation blockage and resulting c-myc deregulation in Burkitt's lymphoma, we analyzed transcription patterns after transfer of normal and Burkitt's lymphoma c-myc alleles into murine cells and Xenopus oocyte germinal vesicles. We have determined that although the mutations within and surrounding several Burkitt's lymphoma c-myc alleles are not sufficient, in themselves, to abrogate the transcription elongation block, transcription initiation from the P2 promoter may be necessary to obtain the block to transcription elongation. To test directly the role of c-myc promoters in programming transcription elongation blockage, we analyzed transcription patterns from in vitro mutagenized c-myc genes containing deletions of either the P1 or P2 promoter. These data confirm that P1-initiated c-myc transcripts do not terminate at discrete sites near the 3' end of exon 1, whereas P2-initiated transcripts either terminate or read through the transcription block signals. Therefore, overexpression and/or constitutive expression from the c-myc P1 promoter may contribute to increased readthrough transcription in Burkitt's lymphoma cells and, hence, to aberrant expression patterns or levels of c-myc steady-state transcripts. In addition, the ability of normal cells to modulate c-myc P2-initiated transcription to either read through or to block elongation provides a fine control mechanism over c-myc steady-state RNA levels.

Germ line c-myc is not down-regulated by loss or exclusion of activating factors in myc-induced macrophage tumors

As in tumors with c-myc chromosomal translocations, c-myc retrovirus-induced monocyte tumors constitutively express an activated form of c-myc (the proviral gene), whereas the normal endogenous c-myc genes are transcriptionally silent. Treatment of these retrovirus-induced tumor cells with a number of bioactive chemicals and growth factors that are known to induce c-myc expression in cells of the monocyte lineage failed to induce the endogenous c-myc gene. In contrast, the same treatments induced the c-fos gene in both tumors and a control macrophage line. To investigate c-myc suppression further, a normal copy of the human c-myc gene was introduced into tumor and control cell lines by using a retrovirus with self-inactivating long terminal repeats. This transduced normal gene was expressed at equivalent levels in all cells, regardless of the state of endogenous c-myc gene expression, and was strongly induced by agents that induce the normal gene in the control cells. These results indicate that the signal transduction pathways that normally activate the c-myc gene are functional in myc-induced tumor cells and suggest that endogenous c-myc is actively suppressed. An examination of the c-myc locus itself showed that the lack of transcriptional activity correlated with the absence of several prominent DNase I-hypersensitive sites in the 5'-flanking region of the gene but without loss of general DNase sensitivity. Furthermore, analysis of 22 methylation-sensitive restriction enzyme sites in the 5'-flanking region, first exon, and first intron indicated that the silent c-myc genes remained in the same unmethylated state as did actively expressed genes. Thus, c-myc suppression does not appear to result from the most frequently described mechanisms of gene inactivation.

Germ line c-myc is not down-regulated by loss or exclusion of activating factors in myc-induced macrophage tumors

As in tumors with c-myc chromosomal translocations, c-myc retrovirus-induced monocyte tumors constitutively express an activated form of c-myc (the proviral gene), whereas the normal endogenous c-myc genes are transcriptionally silent. Treatment of these retrovirus-induced tumor cells with a number of bioactive chemicals and growth factors that are known to induce c-myc expression in cells of the monocyte lineage failed to induce the endogenous c-myc gene. In contrast, the same treatments induced the c-fos gene in both tumors and a control macrophage line. To investigate c-myc suppression further, a normal copy of the human c-myc gene was introduced into tumor and control cell lines by using a retrovirus with self-inactivating long terminal repeats. This transduced normal gene was expressed at equivalent levels in all cells, regardless of the state of endogenous c-myc gene expression, and was strongly induced by agents that induce the normal gene in the control cells. These results indicate that the signal transduction pathways that normally activate the c-myc gene are functional in myc-induced tumor cells and suggest that endogenous c-myc is actively suppressed. An examination of the c-myc locus itself showed that the lack of transcriptional activity correlated with the absence of several prominent DNase I-hypersensitive sites in the 5'-flanking region of the gene but without loss of general DNase sensitivity. Furthermore, analysis of 22 methylation-sensitive restriction enzyme sites in the 5'-flanking region, first exon, and first intron indicated that the silent c-myc genes remained in the same unmethylated state as did actively expressed genes. Thus, c-myc suppression does not appear to result from the most frequently described mechanisms of gene inactivation.

c-myc can induce expression of G0/G1 transition genes

The human c-myc oncogene was linked to the heat shock-inducible Drosophila hsp70 promoter and used to stably transfect mouse BALB/c 3T3 cells. Heat shock of the transfectants at 42 degrees C followed by recovery at 37 degrees C resulted in the appearance of the human c-myc protein which was appropriately localized to the nuclear fraction. Two-dimensional analysis of the proteins of density-arrested cells which had been heat shock treated revealed the induction of eight protein species and the repression of five protein species. All of the induced and repressed proteins were nonabundant. cDNA clones corresponding to genes induced during the G0/G1 transition were used as probes to assay for c-myc inducibility of these genes. Two anonymous sequences previously identified as serum inducible (3CH77 and 3CH92) were induced when c-myc was expressed. In response to serum stimulation, 3CH77 and 3CH92 were expressed before c-myc mRNA levels increased. However, in response to specific induction of c-myc by heat shock of serum arrested cells, 3CH77 and 3CH92 mRNA levels increased after the rise in c-myc mRNA. Therefore, we hypothesize that abnormal expression of c-myc can induce genes involved in the proliferative response.

T cell receptor gene rearrangement and expression in ataxia-telangiectasia B lymphoblastoid cells

Immunoglobulin and T cell receptor gene probes have been used to investigate cell lineage and monoclonality in lymphoid malignancies. In the present study we have used T cell receptor beta- and gamma-chain gene probes to screen for abnormal rearrangements of these genes in B lymphoblastoid cells from patients with ataxia-telangiectasia (A-T). No rearrangement of either gene was observed but deletion of a beta-chain gene allele is described for one A-T cell line. Expression of mRNA hybridizing to the beta-chain gene probe was demonstrated for two A-T homozygotes (brother and sister) as well as for their mother (heterozygote). This transcript was found to be truncated in all three cases.

c-myc can induce expression of G0/G1 transition genes

The human c-myc oncogene was linked to the heat shock-inducible Drosophila hsp70 promoter and used to stably transfect mouse BALB/c 3T3 cells. Heat shock of the transfectants at 42 degrees C followed by recovery at 37 degrees C resulted in the appearance of the human c-myc protein which was appropriately localized to the nuclear fraction. Two-dimensional analysis of the proteins of density-arrested cells which had been heat shock treated revealed the induction of eight protein species and the repression of five protein species. All of the induced and repressed proteins were nonabundant. cDNA clones corresponding to genes induced during the G0/G1 transition were used as probes to assay for c-myc inducibility of these genes. Two anonymous sequences previously identified as serum inducible (3CH77 and 3CH92) were induced when c-myc was expressed. In response to serum stimulation, 3CH77 and 3CH92 were expressed before c-myc mRNA levels increased. However, in response to specific induction of c-myc by heat shock of serum arrested cells, 3CH77 and 3CH92 mRNA levels increased after the rise in c-myc mRNA. Therefore, we hypothesize that abnormal expression of c-myc can induce genes involved in the proliferative response.

Suppression of tumorigenicity in T-cell lymphoma hybrids is correlated with changes in myc expression and DNA replication of the myc chromosomal domain

Intraspecific somatic cell hybrids between T-lymphoma cells and lymphocytes are highly tumorigenic whereas fusion of T-lymphoma cells with normal fibroblasts leads to reduced or even completely suppressed tumorigenicity of the hybrid cells. A particular cytogenetic phenomenon defines these two classes of hybrids. DNA replication analysis via bromodeoxyuridine pulse labelling reveals an aberrant banding pattern in the c-myc chromosomal domain in tumour cells and highly tumorigenic hybrids. In hybrids with suppressed tumorigenicity the tumor parent derived chromosomes have reverted to normal DNA replication banding. Aberrant DNA replication in tumour cells and highly tumorigenic hybrids coincides with enhanced c-myc expression. In hybrids with suppressed tumorigenicity and with normal DNA replication banding c-myc expression is also reduced. Thus, a correlation between aberrant DNA replication and enhanced expression of a gene located in the same chromosomal domain is observed. Reversion of aberrant DNA replication and reduction of c-myc expression to normal in hybrid cells may be due to a site-specific trans effect which overrides the control brought about in cis by retroviral insertion near the c-myc gene.

Strong transcriptional activation of translocated c-myc genes occurs without a strong nearby enhancer or promoter

We have studied the transcriptional activation of translocated c-myc genes in murine plasmacytomas in which the translocation juncture occurs within the first intron of c-myc and juxtaposes c-myc with the immunoglobulin C alpha gene segment. It has been widely suggested that a novel transcriptional enhancer element located near the C alpha gene segment might activate the translocated c-myc gene. We have carried out an extensive search for such an element and find no significant transcriptional enhancer activity in a 22 kb region encompassing the translocation junction, C alpha gene segment and regions 3' of C alpha. We also find that the cryptic promoter region of the translocated c-myc gene is a very weak promoter of transcription. Despite this evidence against the presence of strong transcriptional regulatory elements, the translocated c-myc gene locus is transcribed at high rates that are 25-greater than 100% of that measured for the highly active immunoglobulin genes in murine plasmacytomas. These data suggest the presence of a novel type of strong activator of transcription in the murine heavy chain locus.

Differential promoter utilization by the c-myc gene in mitogen- and interleukin-2-stimulated human lymphocytes

Transcription of the c-myc gene is initiated from two principal promoters, P1 and P2. We demonstrate here that a shift in promoter utilization occurred with time in human peripheral blood mononuclear cells (PBMC) that had been stimulated to proliferate. The P1/P2 ratio reached a maximum of approximately 1.3 at 4 h after phytohemagglutinin stimulation and a minimum of 0.31 at 48 h. Actinomycin decay experiments demonstrated that both P1 and P2 transcripts had similar half-lives at early and late times after mitogen stimulation, indicating that the shift in promoter utilization was probably not posttranscriptionally regulated. Addition of interleukin-2 to previously activated PBMC increased c-myc mRNA, but unlike increases after mitogen stimulation, the P1/P2 ratio stayed less than 0.5. Our findings demonstrated that there was a difference between mitogen- and interleukin-2-stimulated increases in c-myc RNA in PBMC.

Differential promoter utilization by the c-myc gene in mitogen- and interleukin-2-stimulated human lymphocytes

Transcription of the c-myc gene is initiated from two principal promoters, P1 and P2. We demonstrate here that a shift in promoter utilization occurred with time in human peripheral blood mononuclear cells (PBMC) that had been stimulated to proliferate. The P1/P2 ratio reached a maximum of approximately 1.3 at 4 h after phytohemagglutinin stimulation and a minimum of 0.31 at 48 h. Actinomycin decay experiments demonstrated that both P1 and P2 transcripts had similar half-lives at early and late times after mitogen stimulation, indicating that the shift in promoter utilization was probably not posttranscriptionally regulated. Addition of interleukin-2 to previously activated PBMC increased c-myc mRNA, but unlike increases after mitogen stimulation, the P1/P2 ratio stayed less than 0.5. Our findings demonstrated that there was a difference between mitogen- and interleukin-2-stimulated increases in c-myc RNA in PBMC.

Suppression of transformed phenotypes in intraspecific somatic cell hybrid clones between the c-myc activating mouse plasmacytoma line and normal cells

The role of normal-cell-derived chromosome 15 in suppressing transformed phenotypes was studied in intraspecific hybrid clones between the c-myc oncogene activating BALB/c mouse plasmacytoma (S194) cells and normal spleen cells or fibroblasts from CBA/H-T6 mice. All the hybrid clones between S194 and normal spleen cells grew very rapidly in suspension and formed colonies in soft agar. In contrast, the hybrid clones between S194 and normal fibroblasts grew slowly in an attached form. They were divided into 2 groups on the basis of their morphology and growth properties: most clones showed flat type morphology, and no colony formation was seen in soft agar, while some clones grew in a piled-up fashion and formed colonies in soft agar. The hybrid clones between S194 and normal spleen cells lost some normal-cell-derived chromosomes but retained most tumor-derived marker chromosomes including the t(12;15) chromosome which carried the activated c-myc oncogene. On the other hand, hybrid clones between S194 cells and normal fibroblasts retained almost all chromosomes from both parental cells. With respect to retention of normal-cell-derived chromosome 15, both the flat and piled-up type clones retained 2 copies each of the t(14;15) and T6 marker chromosomes, the normal counterparts of the t(12;15) chromosomes. Our results suggest that the transformed phenotypes of the hybrid clones between S194 cells and normal fibroblasts are negatively modulated by normal-cell-derived chromosomes but not by normal-cell-derived chromosome 15 alone.

An active chromatin structure acquired by translocated c-myc genes

We used general sensitivity to DNase I digestion to analyze the chromatin structure of c-myc genes in seven murine plasmacytomas. In every case, the 3' portion of c-myc juxtaposed with C alpha displayed a much more DNase I-sensitive chromatin structure than untranslocated c-myc or, in one case analyzed, the reciprocally translocated 5' portion. Our data suggest the presence of regulatory sequences near the C alpha gene segment.

Mapping by chromosome sorting of several gene probes, including c-myc, to the derivative chromosomes of a 3;8 translocation associated with familial renal cancer

In eight members of a single family a constitutional translocation t(3;8) (p14.2;q24.1) is associated with the development of renal cancer. Chromosomes isolated from a cell line established from a subject with this translocation were analysed in flow with a fluorescence-activated cell sorter (FACS II). Nearly six million chromosomes from the flow karyotype region containing the der(8) and 5.5 million from the region containing the der(3) were sorted, the DNA extracted, digested with EcoRI, size fractionated by electrophoresis, and transferred to nitrocellulose. Hybridization with gene probes for c-mos, which has been localized to 8q11-q22 and somatostatin, which has been mapped to 3q28, confirmed that the sorted fractions contained, respectively, the der(8) and der(3) chromosomes. The cellular oncogenes c-raf-1 (3p25) and c-myc (8q24) were found to be translocated to the der(8) and der(3) chromosomes, respectively. The possible role that the relocation of c-myc might have on the development of renal carcinoma in carriers of this 3;8 translocation was further studied by analysis of the region surrounding the c-myc gene. By the use of cosmid cloning, no rearrangement 31 Kb 5'(or 19 Kb 3') of the translocated gene was found, indicating that the break-point is not immediately adjacent to c-myc. In an associated study, the DNA fragment D3S2 from chromosome 3 was found to map to 3p14.2-pter. This assignment in conjunction with published somatic cell hybrid data enabled D3S2 to be mapped more precisely to the interval 3p14.2-3p21.

Deregulation of c-myc gene expression in human colon carcinoma is not accompanied by amplification or rearrangement of the gene

The structure and expression of the c-myc oncogene were examined in 29 primary human colon adenocarcinomas. Dot blot hybridization of total RNA showed that 21 tumors (72%) had considerably elevated expression of c-myc (5- to 40-fold) relative to normal colonic mucosa. These data were corroborated by Northern blots of polyadenylated RNA, which showed a 2.3-kilobase transcript. Southern analysis of the c-myc locus in these tumors indicated the absence of amplification or DNA rearrangement in a 35-kilobase region encompassing the gene. In a parallel study, elevated expression of c-myc without amplification or DNA rearrangement was also observed in three of six colon carcinoma cell lines examined; in addition, unlike a normal colon cell line control, these three cell lines exhibited constitutive, high-level expression of the gene during their growth in cultures. These results indicate that elevated expression of the c-myc oncogene occurs frequently in primary human colon carcinomas and that the mechanism involved in the regulation of c-myc expression is altered in tumor-derived cell lines.

Analysis of the 3' flanking region of the human c-myc gene in lymphomas with the t(8;22) and t(2;8) chromosomal translocations

We have cloned and mapped the sequences extending 38 kb 3' of the c-myc gene. This region is found to be highly repetitive in nature and hybridizes extensively with a BLUR 8 Alu probe. Unique sequence probes derived from this region were used to map the chromosomal breakpoints of a number of lymphoma cell lines with t(2;8) or t(8;22) translocations. In five of the cell lines (PA682, LY67, LY47, LY66 and LY91), the immunoglobulin light chain locus translocates into a region which is greater than 47 kb downstream of c-myc. For one of the cell lines, JI, the location of the breakpoint on the 8q+ chromosome was found to be 25-32 kb 3' of c-myc. The breakpoint for the BL2 cell line had been previously mapped at 10 kb 3' of the c-myc oncogene. Analyses of steady-state levels of c-myc mRNA in cell lines with chromosomal breakpoints ranging from 10 kb to greater than 47 kb 3' of c-myc range from 0.5 to 10X the levels in lymphoblast controls. The different levels of c-myc transcripts is not a direct function of the distance between the c-myc gene and the translocated immunoglobulin light chain locus.

An active chromatin structure acquired by translocated c-myc genes

We used general sensitivity to DNase I digestion to analyze the chromatin structure of c-myc genes in seven murine plasmacytomas. In every case, the 3' portion of c-myc juxtaposed with C alpha displayed a much more DNase I-sensitive chromatin structure than untranslocated c-myc or, in one case analyzed, the reciprocally translocated 5' portion. Our data suggest the presence of regulatory sequences near the C alpha gene segment.

Regulation of translocated c-myc genes transfected into plasmacytoma cells

We have transfected two translocated c-myc oncogene clones, derived from two human lymphomas carrying the t(8;14) chromosome translocation, into mouse plasmacytoma cells to study the regulation of their expression. In one case, the transfected clone contained the two coding exons of the c-myc oncogene translocated to an immunoglobulin heavy-chain switch region; in the other case, the two coding exons were translocated 5' of the enhancer element located between the heavy-chain joining region (JH) and the switch region S mu. Nuclease S1 protection experiments indicate that only the c-myc translocated 5' of the enhancer element is transcribed in the plasmacytoma cells. Thus, 5'-truncation of the c-myc gene per se does not lead to c-myc deregulation. Further, since the level of c-myc transcripts in the parental human lymphoma cells was 3- to 4-fold higher than in the transfectants, it seems likely that additional elements within the heavy-chain locus may play a role in the enhancement of c-myc gene transcription in lymphoma cells.

Expression of a translocated c-abl gene in hybrids of mouse fibroblasts and chronic myelogenous leukaemia cells

Chronic myelogenous leukaemia (CML) is a clonal disease arising from malignant transformation of pluripotent hematopoietic stem cells. In most cases, it is characterized by the presence of the Philadelphia (Ph1) chromosome (22q-) which results from a reciprocal translocation between chromosomes 9 and 22 (refs 1-3). In this translocation, the human homologue of the Abelson virus oncogene, c-abl, normally on chromosome 9, is moved to chromosome 22, while c-sis, the cellular homologue of the simian sarcoma virus oncogene, is moved from chromosome 22 to chromosome 9 (refs 4-6). CML cells carrying the t(9;22) chromosomal translocation are known to produce an 8-kilobase (kb) c-abl transcript in addition to the normal 6- and 7-kb transcripts and to express the normal p145 abl protein and a p210 c-abl protein possessing a tyrosine kinase activity not detected in the p145 species. Results of our analyses using somatic cell hybrids between a mouse fibroblast line and two human CML-derived cell lines which carry the Ph1 chromosome and are phenotypically identical to the fibroblast parent indicate that only the hybrid cells containing Ph1 chromosome express both the 8-kb c-abl RNA and the p210 protein. Thus, expression of the altered c-abl transcripts and protein depends on the presence of the Ph1 chromosome and is not myeloid-specific.

The t(14;18) chromosome translocations involved in B-cell neoplasms result from mistakes in VDJ joining

In this study, the joining sequences between chromosomes 14 and 18 on the 14q+ chromosomes of a patient with pre-B-cell leukemia and four patients with follicular lymphoma carrying a t(14;18) chromosome translocation were analyzed. In each case, the involved segment of chromosome 18 has recombined with the immunoglobulin heavy-chain joining segment (JH) on chromosome 14. The sites of the recombination on chromosome 14 are located close to the 5' end of the involved JH segment, where the diversity (D) regions are rearranged with the JH segments in the production of active heavy-chain genes. As extraneous nucleotides (N regions) were observed at joining sites and specific signal-like sequences were detected on chromosome 18 in close proximity to the breakpoints, it is concluded that the t(14;18) chromosome translocation is the result of a mistake during the process of VDJ joining at the pre-B-cell stage of differentiation. The putative recombinase joins separated DNA segments on two different chromosomes instead of joining separated segments on the same chromosome, causing a t(14;18) chromosome translocation in the involved B cells.

The role of the c-mos gene in the 8;21 translocation in human acute myeloblastic leukemia

The human c-mos proto-oncogene is located on chromosome 8 at band q22, close to the breakpoint in the t(8;21) (q22;q22) chromosome rearrangement. This translocation is associated with acute myeloblastic leukemia, subgroup M2. The c-myc gene, another proto-oncogene, has been mapped to 8q24. The breakpoint at 8q22 separates these genes, as determined by in situ hybridization of c-mos and c-myc probes. The c-mos gene remains on the 8q-chromosome and the c-myc gene is translocated to the 21q+ chromosome. Southern blot analysis of DNA from bone marrow cells of four patients with this translocation showed no rearrangement of c-mos.

Deregulation of c-myc gene expression in human colon carcinoma is not accompanied by amplification or rearrangement of the gene

The structure and expression of the c-myc oncogene were examined in 29 primary human colon adenocarcinomas. Dot blot hybridization of total RNA showed that 21 tumors (72%) had considerably elevated expression of c-myc (5- to 40-fold) relative to normal colonic mucosa. These data were corroborated by Northern blots of polyadenylated RNA, which showed a 2.3-kilobase transcript. Southern analysis of the c-myc locus in these tumors indicated the absence of amplification or DNA rearrangement in a 35-kilobase region encompassing the gene. In a parallel study, elevated expression of c-myc without amplification or DNA rearrangement was also observed in three of six colon carcinoma cell lines examined; in addition, unlike a normal colon cell line control, these three cell lines exhibited constitutive, high-level expression of the gene during their growth in cultures. These results indicate that elevated expression of the c-myc oncogene occurs frequently in primary human colon carcinomas and that the mechanism involved in the regulation of c-myc expression is altered in tumor-derived cell lines.

Localization of the oncogene c-erbA1 immediately proximal to the acute promyelocytic leukaemia breakpoint on chromosome 17

Using in situ hybridization, c-erbA1 has been mapped immediately distal to the translocation breakpoint on chromosome 17 in fibroblasts with a karyotype 46,XX, t(15;17)(q22;q11). Previous work has shown that c-erbA1 is proximal to the translocation breakpoint on chromosome 17 in the t(15;17)(q22;q12-21) in acute promyelocytic leukaemia. The oncogene can therefore be localized to the region of chromosome 17 between the breakpoints in 17q11 and 17q12-21.

Cloning and sequencing of a c-myc oncogene in a Burkitt's lymphoma cell line that is translocated to a germ line alpha switch region

We have cloned and sequenced the translocated c-myc gene from the Burkitt's lymphoma CA46 cell line that carries a reciprocal translocation between chromosomes 8 and 14. The breakpoint lies within the first intron of c-myc, so that the first noncoding exon of the gene remains on the 8q- chromosome. The second and third coding exons are translocated to the 14q+ chromosome into the switch region of C-alpha 1. The orientation of the c-myc gene with relationship to alpha 1 is 5' to 5', with directions of transcription in opposite orientation. DNA sequencing studies predict five changes in the amino acid sequence of the myc protein, two of which occur in a region within the second exon which is highly conserved in evolution. Southern blotting data indicate that the first exon of c-myc is rearranged 3' to 3' with the pseudo-epsilon gene. Because CA46 cells contain two rearranged mu genes, the translocation must have occurred after immunoglobulin rearrangement. The position of the breakpoint in CA46 occurs within a 20-base-pair region of the first intron of c-myc to which breakpoints have been mapped for two additional B-cell lymphomas with the t(8;14) translocation, ST486 and the Manca cell line. The region of the heavy chain locus to which c-myc has translocated is different in each case. Comparisons have been made of the levels of transcripts of the translocated c-myc gene in ST486 and CA46, where the gene is not associated with the heavy chain enhancer, with its expression in the Manca cell, in which it is. The c-myc gene is transcribed at similar levels in all three cases.

The translocated c-myc oncogene of Raji Burkitt lymphoma cells is not expressed in human lymphoblastoid cells

We hybridized Raji Burkitt lymphoma cells, which carry a t(8;14) chromosome translocation, with human lymphoblastoid cells to study the expression of the translocated cellular myc oncogene (c-myc) in the hybrid cells. In Raji cells the c-myc oncogene is translocated to a switch region of the gamma heavy chain locus (S gamma). Because of sequence alterations in the 5' exon of the translocated c-myc oncogene in this cell line, it is possible to distinguish the transcripts of the translocated c-myc gene and of the normal c-myc gene. S1 nuclease protection experiments with a c-myc first exon probe indicate that Raji cells express predominantly the translocated c-myc gene, while the level of expression of the normal c-myc gene is less than 2% of that of the translocated c-myc gene. Somatic cell hybrids between Raji and human lymphoblastoid cells retain the lymphoblastoid phenotype and express only the normal c-myc oncogene. This result indicates that the activation of a c-myc oncogene translocated to a S region depends on the stage of B-cell differentiation of the cells harboring the translocated c-myc gene and not on alterations in the structure of the translocated c-myc oncogene.

Genetics of B-cell neoplasia

Somatic cell hybridization experiments between tumorigenic and nontumorigenic cells indicate that the differentiated state of the cells carrying the activated oncogene is important in the regulation of their expression.

Cloning and sequencing of a c-myc oncogene in a Burkitt's lymphoma cell line that is translocated to a germ line alpha switch region

We have cloned and sequenced the translocated c-myc gene from the Burkitt's lymphoma CA46 cell line that carries a reciprocal translocation between chromosomes 8 and 14. The breakpoint lies within the first intron of c-myc, so that the first noncoding exon of the gene remains on the 8q- chromosome. The second and third coding exons are translocated to the 14q+ chromosome into the switch region of C-alpha 1. The orientation of the c-myc gene with relationship to alpha 1 is 5' to 5', with directions of transcription in opposite orientation. DNA sequencing studies predict five changes in the amino acid sequence of the myc protein, two of which occur in a region within the second exon which is highly conserved in evolution. Southern blotting data indicate that the first exon of c-myc is rearranged 3' to 3' with the pseudo-epsilon gene. Because CA46 cells contain two rearranged mu genes, the translocation must have occurred after immunoglobulin rearrangement. The position of the breakpoint in CA46 occurs within a 20-base-pair region of the first intron of c-myc to which breakpoints have been mapped for two additional B-cell lymphomas with the t(8;14) translocation, ST486 and the Manca cell line. The region of the heavy chain locus to which c-myc has translocated is different in each case. Comparisons have been made of the levels of transcripts of the translocated c-myc gene in ST486 and CA46, where the gene is not associated with the heavy chain enhancer, with its expression in the Manca cell, in which it is. The c-myc gene is transcribed at similar levels in all three cases.

"Retroposon" insertion into the cellular oncogene c-myc in canine transmissible venereal tumor

We examined by Southern blotting the state of the cellular oncogene c-myc in the dog transmissible venereal tumor. The tumor DNA contains a 16.8-kilobase pair (kbp) rearranged c-myc fragment in addition to the normal 15-kbp and 7.5-kbp fragments. We compared the structure of the cloned rearranged c-myc (re-myc) with that of a cloned normal c-myc and found that the rearrangement was due to the insertion of a 1.8-kbp DNA upstream to the first exon of c-myc. The inserted DNA is flanked by 10-base-pair direct repeats and contains a dA-rich tail, suggesting its origin from mRNA. Partial sequence of the inserted element showed 62% homology with the primate interdispersed Kpn I repetitive element. These results provide an example for the behavior of repetitive DNA sequences like the Kpn I family, as movable elements that can transpose nearby to oncogenes or other structural genes and perhaps affect their activity.

Cloning of the chromosome breakpoint of neoplastic B cells with the t(14;18) chromosome translocation

From an acute B-cell leukemia cell line, a DNA probe was obtained that was specific for chromosome 18 and flanked the heavy chain joining region of the immunoglobulin heavy chain locus on chromosome 14. This probe detected rearrangement of the homologous DNA segment in the leukemic cells and in follicular lymphoma cells with the t(14:18) chromosome translocation but not in other neoplastic or normal B or T cells. The probe appears to identify bcl-2, a gene locus on chromosome 18 (band q21) that is unrelated to known oncogenes and may be important in the pathogenesis of B-cell neoplasms with this translocation.

A 14;18 and an 8;14 chromosome translocation in a cell line derived from an acute B-cell leukemia

We have established a cell line, which we named 380, from a young male with acute lymphoblastic leukemia (FAB type L2). Karyologic analysis of this cell line indicates that it carries an 8;14 and a 14;18 chromosome translocation, which are characteristic of Burkitt lymphoma and of follicular lymphoma, respectively. This cell line is Epstein-Barr virus antigen-negative, reacts with monoclonal antibodies specific for B cells, and contains rearranged immunoglobulin heavy and light chain genes, but does not express human immunoglobulins. In this cell line, both mu heavy chain constant (C mu) loci are rearranged within the joining (JH) DNA segment. One of the JH segments on one of the 14q+ chromosomes is rearranged with a segment of chromosome 8, where the c-myc oncogene resides, while the other is rearranged with a segment of chromosome 18 where a putative oncogene, which we have called bcl-2, is located. The c-myc oncogene, which is translocated to one of the 14q+ chromosomes, is in its germ-line configuration more than 14 kilobases away from both the JH segment and the heavy chain enhancer that is located between the JH and mu switch region. Based on these findings, we propose a model of some aspects of B-cell oncogenesis according to which B-cell neoplasms carrying translocations involving the heavy chain loci on both human chromosomes 14 are the result of a multiple step process.