Partial repression of human gamma-globin genes by LCR element HS3 when linked to beta-globin genes and LCR element...

Clues for overcoming fetal (gamma-) globin gene repression in adult human erythroid cells may come from understanding why repression of isolated gamma-globin genes has not previously been achieved in the adult erythroid environment of mouse erythroleukemia cells (MEL). Repression of human gamma-globin genes has been demonstrated in MEL cells when transferred as part of the entire beta-globin gene cluster packaged in chromatin. Major differences in these approaches are prior packaging into chromatin and the presence of additional sequences, notably from the locus control region (LCR). In this report we focus on the contribution to gamma-globin gene repression that multiple elements of the LCR may have. We first show preferential activation of beta-globin genes over gamma-globin genes in MEL cells when linked to each other and to LCR sequences containing the core elements of DNase I hypersensitive sites 4, 3, and 2. Removal of the HS4 element had no effect, however, removal of the 225 bp HS3 core element resulted in a five-fold increase in gamma-globin gene expression. The enhancer 3' to the A gamma-globin gene also had no apparent effect on gamma-globin gene expression. These results provide first evidence of gamma-globin gene repression involving the core region of HS3 in the presence of the core region of HS2 and a beta-globin gene. A mechanism for repression involving sequestration of the gamma-promoter away from the strong enhancer activity of HS2 is proposed.