The chicken beta-globin 5 ' HS4 boundary element blocks enhancer-mediated suppression of silencing

A constitutive DNase I-hypersensitive site 5' of the chicken beta-globin lo cus, termed 5'HS4 or cHS4, has been shown to insulate a promoter from the e ffect of an upstream enhancer and to reduce position effects on mini-white expression in Drosophila cells; on the basis of these findings, it has been designated a chromatin insulator. We have examined the effect of the cHS4 insulator in a system that assays both the level of gene expression and the rate of transcriptional silencing. Because transgenes flanked by insulator elements are shielded from position effects in Drosophila cells, we tested the ability of cHS4 to protect transgenes from position effects in mammali an cells. Flanking of an expression vector with the cHS4 insulator in a col ony assay did not increase the number of G418-resistant colonies. Using lox /cre-based recombinase-mediated cassette exchange to control integration po sition, we studied the effect of cHS4 on the silencing of an integrated bet a-geo reporter at three genomic sites in K562 erythroleukemia cells. In thi s assay, enhancers act to suppress silencing but do not increase expression levels. While cHS4 blocked enhancement at each integration site, the stren gth of the effect varied from site to site. Furthermore, at some sites, cHS 4 inhibited the enhancer effect either when placed between the enhancer and the promoter or when placed upstream of the enhancer. These results sugges t that the activity of cHS4 is not dominant in all contexts and is unlikely to prevent silencing at all genomic integration sites.