CpG Islands from the α-Globin Gene Cluster Increase Gene Expression in an Integration-Dependent Manner

Abstract

In contrast to other globin genes, the human and rabbit α-globin genes are expressed in transfected erythroid and nonerythroid cells in the absence of an enhancer. This enhancer-independent expression of the α-globin gene requires extensive sequences not only from the 5′ flanking sequence but also from the intragenic region. However, the features of these internal sequences that are responsible for their positive effect are unclear. We tested several possible determinants of this activity. One possibility is that a previously identified array of discrete binding sites for known and potential regulatory proteins within the α-globin gene comprise an intragenic enhancer specific for the α-globin promoter, but directed rearrangements of the sequences show that this is not the case. Alternatively, the promoter may extend into the gene, with the function of the discrete binding sites being dependent on maintenance of their proper positions and orientations relative to the 5′ flanking sequence. However, the positive effects observed in gene fusions do not localize to a discrete region of the α-globin gene and the results of internal deletions and point mutations argue against a required role of the targeted discrete binding sites. A third possibility is that the CpG island, which includes both the 5′ flanking and intragenic regions associated with the positive activity, may itself have a more general effect on expression in transfected cells. Indeed, we show that the size of the CpG island in constructs correlates with the level of gene expression. Furthermore, the α-globin promoter is more active in the context of a previously inactive CpG island than in an A+T-rich context, showing that the CpG island provides an environment more permissive for expression. These effects are seen only after integration, suggesting a possible mechanism at the level of chromatin structure.