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

Abstract

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 3′Cα 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.