The Immunoglobulin Heavy Chain Locus Control Region Increases Histone Acetylation along Linked c-myc Genes

Abstract

In chromosome translocations characteristic of Burkitt lymphomas (BL) and murine plasmacytomas, c-myc genes become juxtaposed to immunoglobulin heavy-chain (IgH) sequences, resulting in aberrant c-myc transcription. Translocated c-myc alleles that retain the first exon exhibit increased transcription from the normally minor c-myc promoter, P1, and increased transcriptional elongation through inherent pause sites proximal to the major c-myc promoter, P2. We recently demonstrated that a cassette derived from four DNase I-hypersensitive sites (HS1234) in the 3C␣ region of the IgH locus functions as an enhancer-locus control region (LCR) and directs a similar pattern of deregulated expression of linked c-myc genes in BL and plasmacytoma cell lines. Here, we report that the HS1234 enhancer-LCR mediates a widespread increase in histone acetylation along linked c-myc genes in Raji BL cells. Significantly, the increase in acetylation was not restricted to nucleosomes within the promoter region but also was apparent upstream and downstream of the transcription start sites as well as along vector sequences. Histone hyperacetylation of control c-myc genes, which was induced by the deacetylase inhibitor trichostatin A, mimics the effect of the HS1234 enhancer on expression from the c-myc P2 promoter, but not that from the P1 promoter. These results suggest that the HS1234 enhancer stimulates transcription of c-myc by a combination of mechanisms. Whereas HS1234 activates expression from the P2 promoter through a mechanism that includes increased histone acetylation, a general increase in histone acetylation is not sufficient to explain the HS1234-mediated activation of transcription from P1.

The rapid destabilization of p53 mRNA in immortal chicken embryo fibroblast cells

“…To prepare nuclear and cytoplasmic RNA, both fractions were separated as described in nuclear run-on assay (Madisen et al, 1998), and total RNA was isolated using TRIzol. For Northern transfer, membranes were blotted with 20 mg of total RNA extracted from di erent primary and immortal CEF cells then hybridized with [a-32 P]dATP-labeled chicken p53 and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) cDNA fragments derived by reverse transcription polymerase chain reaction (RT ± PCR).…”

“…Nuclei isolated from primary and immortal CEF cells were used to label the de novo synthesized transcripts with [a-32 P]UTP as described previously (Madisen et al, 1998). Nuclear run-on transcription rates were determined by the Figure 7 The half-life of p53 N +LacZ 7N fusion mRNA in primary and immortal CEF cells.…”

NF-κB Activity Is Required for the Deregulation of c-myc Expression by the Immunoglobulin Heavy Chain Enhancer

“…It is of interest to compare our results with MHS4 and the 9 -12), to MSH1234 with the MHS4 NF-B site mutated (columns [13][14][15][16], and with the c-myc promoter with both NF-B sites mutated linked to wild type MHS1234 (columns [17][18][19][20]. For the co-transfection studies, 2 g of c-myc promoter construct was used with 0, 1, 4, or 8 g of the super-repressor IB␣ expression vector in columns 1, 5, 9, 13, and 17; columns 2, 6, 10, 14, and 18; columns 3, 7, 11, 15, and 19; and columns 4, 8, 12, 16, and 20, respectively.…”

“…Recently, it has been shown that MHS1-MHS4 increase expression from the c-myc P2 promoter by an increase in histone acetylation. However, this increase in acetylation does not explain the MHS1-4 activation of transcription from the P1 promoter (16). Enhancers have been located downstream of two human C␣ genes (17)(18)(19), and these regions share some homology with the murine HS1-4, but only limited functional studies have been performed on the human enhancers.…”

The β-globinlocus control region (LCR) functions primarily by enhancing the transition from transcription initiation to elongation

“…Enhancers increase either the rate of transcription, the number of the templates engaged in transcription, or both. Studies using transgenes or in vitro templates have linked enhancer activities to various events such as PIC assembly (Kim et al 1998;Yie et al 1999), histone acetylation at the promoter (Madisen et al 1998), and nuclear localization (Francastel et al 1999). However, it is poorly understood which events in transactivation are the actual targets of long-range enhancer function at native gene loci.…”