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Molecular Biology and Genetics |
Center for Molecular Biology in Medicine, Palo Alto Veterans Affairs Medical Center and Department of Medicine, Stanford University School of Medicine, Stanford, California 94305
| ABSTRACT |
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10% of the transformed lymphomas have a translocation of c-myc in addition to the translocation of bcl-2 found in the original follicular lymphoma. We investigated whether transcriptional deregulation of bcl-2 and c-myc could be examined in primary lymphoma cells by in vivo footprinting and in vitro protein-DNA binding studies. A matched pair of follicular and transformed lymphoma samples was examined. The transformed lymphoma had acquired a translocation of c-myc into the immunoglobulin heavy chain locus. High levels of bcl-2 expression were observed in both the follicular and transformed lymphomas, whereas the expression of c-myc was low in the follicular lymphoma and increased in the transformed lymphoma. In vivo footprint analysis revealed that a CRE site and a Cdx site in the bcl-2 promoter were occupied on the translocated alleles but not on the normal alleles in both the follicular and transformed lymphomas. Two nuclear factor
B sites were occupied on the translocated c-myc allele in the transformed lymphoma. Gel shift analysis revealed that these proteins bound to their respective sites in the bcl-2 or c-myc promoter. There was no evidence that the presence of one of the translocations in the immunoglobulin heavy chain locus influenced the expression of the other translocated gene. | INTRODUCTION |
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Translocation of the c-myc gene to one of the immunoglobulin gene loci is seen in Burkitts lymphoma. Deregulated expression of the translocated c-myc gene is found in Burkitts lymphoma cells, and it is likely that the immunoglobulin enhancers play a role in the deregulated expression of c-myc.
Progression of follicular lymphoma to a more aggressive intermediate or high-grade lymphoma occurs in the majority of patients. The transformed lymphomas retain the t(14;18) translocation and usually acquire multiple, complex new chromosomal abnormalities. A new translocation of the c-myc gene into the immunoglobulin locus is observed in
10% of the transformed lymphomas. Bcl-2 and c-Myc act synergistically in transgenic mice, as noted above (4, 5, 6)
, and also in transfection studies (7)
. Insight into the interaction of Bcl-2 and c-Myc comes from studies demonstrating that Bcl-2 prevents the apoptosis induced by c-Myc but does not interfere with its proliferative effect (8, 9, 10)
.
We have studied the molecular mechanisms of deregulation of the bcl-2 and c-myc genes in follicular and Burkitts lymphoma, respectively. Our studies have identified two positive regulatory transcription factors, CREB and a Cdx/A-Myb complex, that activate the bcl-2 promoter in B cells (11
, 12)
. Two NF-
B sites have been shown to be important for the regulation of both the human (13)
and murine (14
, 15)
c-myc promoters. We have also observed protection in vivo over a site for NM23H2 or a related transcription factor (16)
.
It is not known whether the translocated c-myc gene in a transformed lymphoma is deregulated in a manner similar to that observed in Burkitts lymphoma. We also wished to determine whether the regulation of bcl-2 changed with transformation of the lymphoma or whether expression of bcl-2 was influenced by the presence of the c-myc translocation in the other IgH allele. Previous studies on the deregulation of bcl-2 and c-myc by translocation into the IgH gene have been performed on cell lines. It is important to demonstrate that similar mechanisms of deregulation occur in actual patient samples. We studied matched pairs of follicular and transformed lymphomas to examine the deregulation of the bcl-2 gene. One of the transformed lymphomas demonstrated a translocation of the c-myc gene into the IgH locus. We characterized the c-myc translocation and demonstrated that both bcl-2 and c-myc were expressed at high levels in the transformed lymphoma. Furthermore, we showed by in vivo footprinting and EMSA analysis that CREB and Cdx bind to the bcl-2 promoter and that NF-
B binds to the c-myc promoter.
| MATERIALS AND METHODS |
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4.
Tumor Specimens.
Biopsy specimens from patients with lymphoma were obtained after informed consent using a protocol approved by the human subjects investigations review board. The specimens were stored as frozen viable single-cell suspensions as described previously (17)
. B cells were purified by either positive selection with anti-CD19 or by negative selection. This case was diagnosed as follicular small cleaved cell lymphoma and progressed to a diffuse large cell lymphoma in 6 years. Biopsies from both time points were available for study.
Conventional Gel Electrophoresis.
DNA was extracted by standard techniques and digested with the indicated restriction enzymes. Electrophoresis was performed in 0.8% agarose gels.
Pulsed-Field Electrophoresis.
Cells were embedded in agarose plugs, lysed with detergent, and treated with proteinase K as described (18
, 19)
. NotI digestion was performed, and the samples were separated on a 1% agarose gel in 0.5x Tris-borate-EDTA for 23 h at 160 V (5.5 V/cm) at room temperature with a pulse time of 55 s.
Preparation and Analysis of RNA.
Total cellular RNA was isolated by the guanidinium thiocyanate method, and 8 µg of RNA/sample were separated on 1.2% agarose-formaldehyde gels. The bcl-2 and c-myc probes are described above. Human GAPDH was used to control for variations in sample loading.
Ligation-mediated PCR.
In vivo footprinting by ligation-mediated PCR was performed as described previously (13
, 18
, 20)
. The primers used for PCR were as described by Ji et al. (18)
for the CRE site in the bcl-2 promoter and by Ji et al. (13)
for the NF-
B sites in the c-myc gene. The guanine/adenine-specific cleavage of methylated DNA was performed as described by Strauss et al. (21)
. The primers used for the Cdx site are shown below.
The coding primers were as follows:
5'-TTTCCCCCTTGGCATGAGATG-3'
5'-GATCTTTATTTCATGAGGCACGTTATAGTAAGT-3'
5'-TTCATGAGGCACGTTATAGTAAGTATTTTTAATATC-3'
The noncoding primers were as follows:
5'-GCATTCGAGTAAATTTAATTTCCAGGCAGC-3'
5-TTCCAGGCAGCTTAATACATTCTTTTTAGCC-3'
5'-GGCAGCTTAATACATTCTTTTTAGCCGTGTTAC-3'
EMSA.
The double-stranded oligonucleotides used for EMSA are shown below with the CRE, Cdx, and NF-
B sites in bold:
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The oligonucleotides were synthesized with 5' overhangs and labeled with [
-32P]dCTP and Klenow polymerase. Binding conditions and electrophoresis were as described previously (11
, 13)
.
| RESULTS |
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Northern Analysis of the Follicular and Transformed Lymphomas.
bcl-2 mRNA was observed in both the follicular and transformed lymphomas at a similar level (Fig. 2A)
. Only a low level of c-myc mRNA was found in the follicular lymphoma. In the transformed lymphoma, c-myc expression was considerably higher (Fig. 2B)
. This is consistent with deregulation of c-myc expression associated with translocation into the IgH gene locus.
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B Sites on the Translocated c-myc Allele.
B site in the 5' flanking region. Three guanine residues were protected on the coding strand, and two guanines were protected on the noncoding strand (Fig. 7A)
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B site on the translocated c-myc allele. Three guanines were protected on the coding strand, and two guanine residues showed protection on the noncoding strand (Fig. 7B)
NF-
B Proteins Present in the Lymphoma Cells Bind to the c-myc NF-
B Sites.
EMSA was performed with oligonucleotides that encompassed the 5' flanking and the exon NF-
B sites. The results with the exon NF-
B site oligonucleotide are shown in Fig. 8
, and the results with the 5' flanking probe were similar. Although no binding of NF-
B to the 5' flanking or exon NF-
B sites in the follicular lymphoma could be demonstrated by in vivo footprint analysis, NF-
B proteins are present in nuclear extracts from the follicular lymphoma. One intense complex was observed with the NF-
B site, and faint complexes of slower and faster mobility could be seen as well (Fig. 8
, Lanes 1 and 2). An antibody against p50 supershifted all of the complexes, although the intense complex was not completely shifted (Fig. 8
, Lane 3). Excess cold consensus NF-
B and exon NF-
B oligonucleotides prevented complex formation (Fig. 8
, Lanes 4 and 5). The c-myc 5' flanking NF-
B oligonucleotide also prevented complex formation when it was present in excess (data not shown).
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| DISCUSSION |
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We previously have shown that the CRE site in the bcl-2 promoter is occupied on the translocated allele in the DHL-4 cell line and that this site is a positive regulatory element (12 , 18) . The CRE site was occupied on the translocated bcl-2 gene in the primary follicular and transformed lymphoma cells. This finding suggests that the CRE site plays a role in the regulation of bcl-2 expression in primary lymphoma cells that have not been adapted to growth in tissue culture. We also showed that CREB proteins were present in the nuclear extracts of both the follicular and transformed lymphomas and that they bound to the bcl-2 promoter CRE site. A-Myb and Cdx cooperate to increase bcl-2 promoter activity in B cells (11) . Because the Cdx site contains only a single guanine, with the standard methylation protection assay we have not previously observed in vivo protection of this site. Occupation of the Cdx site on the translocated bcl-2 allele was observed in both the follicular and transformed lymphoma cells with a modification of the in vivo footprinting procedure that allows analysis of both guanine and adenine residues. Our studies suggest that both CREB and the Cdx/A-Myb complex are important for regulation of the translocated bcl-2 allele in primary lymphoma cells.
In Burkitts cell lines, we have shown that two NF-
B sites and a binding site for NM23 or a related transcription factor are occupied on the translocated c-myc allele but not on the normal allele (13
, 16)
. It is important to examine the regulation of the c-myc gene in primary cells because it is possible that this gene product plays a role in adaptation of cells to growth in tissue culture. No clear footprints were observed over these sites in the c-myc promoter in the follicular lymphoma cells. This may reflect the fact that c-myc expression is very low in these cells. We did observe footprints over both NF-
B sites on the translocated c-myc allele in the transformed lymphoma cells. There was no footprint over the NM23 site in these cells. It is possible that this reflects a difference in the regulation of c-myc expression in the primary lymphoma cells compared with the Burkitts lymphoma cell lines. It is also possible that the activity of two NF-
B sites is sufficient for expression of c-myc. In the Burkitts cell lines that we studied previously, only one NF-
B site was present on the translocated c-myc allele. In the Raji cell line, the exon NF-
B site was deleted, whereas the 5' flanking NF-
B site was removed by the translocation in the Ramos cell line.
The bcl-2 gene was expressed at high levels in both the follicular and transformed lymphomas. It appears that translocation of the c-myc gene into the other IgH locus has no effect on the expression of the translocated bcl-2 gene. Deregulation of bcl-2 and c-myc expression by translocation is thought to be attributable to the influence of the IgH enhancers and interactions of these elements with the bcl-2 and c-myc promoters. We have preliminary evidence that overlapping, but not identical, elements of the IgH enhancers are required for deregulation of the bcl-2 and c-myc genes. The regulatory elements in the enhancers that deregulate c-myc or bcl-2 also differ somewhat from those that control expression of the normal IgH gene (29) .4
From these studies we can conclude that the deregulation of the bcl-2 gene in the t(14;18) translocation is similar in follicular and transformed lymphomas. A similar pattern of deregulation of c-myc expression was observed in the transformed lymphoma and in Burkitts cell lines. We did not find any evidence to suggest that the presence of one translocation had any effect on expression of the gene involved in the other translocation. Primary lymphoma samples can be used to study the regulation of the translocated bcl-2 and c-myc genes, and the mechanism of deregulation of each promoter appears to be similar to that observed in established cell lines.
| FOOTNOTES |
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1 Supported by NIH Grants CA69322 and CA56764. ![]()
2 To whom requests for reprints should be addressed, at Division of Hematology, S-161, Stanford University School of Medicine, Stanford, CA 94305-5112. Phone: (650) 849-0551; Fax: (650) 858-3982; E-mail: lboxer{at}stanford.edu ![]()
3 The abbreviations used are: IgH, immunoglobulin heavy chain; CREB, cAMP-responsive element binding protein; NF-
B, nuclear factor
B; EMSA, electrophoretic mobility shift assay; CRE, cAMP-responsive element; GAPDH, glyceraldehyde-3-phosphate dehydrogenase. ![]()
Received 2/ 6/01. Accepted 5/ 1/01.
| REFERENCES |
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B sites function as positive regulators of expression of the translocated c-myc allele in Burkitts lymphoma. Mol. Cell. Biol., 14: 7967-7974, 1994.
B element within exon 1 of the murine c-myc gene. Oncogene, 7: 2447-2453, 1992.[Medline]
B rel family of transcription factors. Mol. Cell. Biol., 14: 1039-1044, 1994.
-globin locus upstream regulatory element by guanine and adenine ligation-mediated polymerase chain reaction. Mol. Cell. Biol., 12: 2135-2142, 1992.
light chain genes in mature B cells in vitro and in vivo. Function of reexpressed recombination-activating gene (RAG) products. J. Exp. Med., 187: 795-799, 1998.
B activity is required for the deregulation of c-myc expression by the immunoglobulin heavy chain enhancer. J. Biol. Chem., 275: 32338-32346, 2000.This article has been cited by other articles:
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