Activation of the beta globin locus by transcription factors and chromatin modifiers

Abstract

Locus control regions (LCRs) alleviate chromatinmediated transcriptional repression. Incomplete LCRs partially lose this property when integrated in transcriptionally restrictive genomic regions such as centromeres. This frequently results in position effect variegation (PEV), i.e. the suppression of expression in a proportion of the cells. Here we show that this PEV is in¯uenced by the heterochromatic protein SUV39H1 and by the Polycomb group proteins M33 and BMI-1. A concentration variation of these proteins modulates the proportion of cells expressing human globins in a locus-dependent manner. Similarly, the transcription factors Sp1 or erythroid Kru È ppel-like factor (EKLF) also in¯uence PEV, characterized by a change in the number of expressing cells and the chromatin structure of the locus. However, in contrast to results obtained in a euchromatic locus, EKLF in¯uences the expression of the g-more than the b-globin genes, suggesting that the relief of silencing is caused by the binding of EKLF to the LCR and that genes at an LCR proximal position are more likely to be in an open chromatin state than genes at a distal position. Keywords: gene activation/position effects/transcription IntroductionThe observation that the presence of the b globin locus control region (LCR) results in copy number-dependent expression of a globin transgene, independent of its position of integration in the host genome, led to the proposal that LCR function includes the establishment and/or maintenance of an open chromatin structure (Grosveld et al., 1987;Festenstein et al., 1996;Milot et al., 1996). Partial LCRs have lost this property and are very sensitive to position effects, particularly when integrated into transcriptionally repressive regions in the genome of transgenic mice (reviewed in Kioussis and Festenstein, 1997). Such integration leads to expression in only a proportion of the cells, which is similar to classical position effect variegation (PEV) (reviewed in Karpen, 1994) or to a novel cell timing position effect, in which all cells express but for only part of the time (Milot et al., 1996). The PEV expression was found in mice containing a deletion of the hypersensitive site 2 (HS2) of the b globin LCR, but only when the transgene was integrated in transcriptionally repressive regions such as found in centromeres (Milot et al., 1996). The subpopulation of erythroid cells that did not transcribe the human b globin gene was associated with a loss of DNase I hypersensitivity within the human b globin locus. Very similar results were obtained in other studies involving partial LCRs (Robertson et al., 1995;Festenstein et al., 1996;Boyer et al., 1997;Guy et al., 1997).Many transcription factors bind to the human b globin locus, but their role in transcriptional activation is not yet clear. An exception is the erythroid-speci®c erythroid Kru Èppel-like factor (EKLF) , which is essential for adult b globin gene expression (Nuez et al., 1995;Perkins et al., 1996;Wijgerde et al., 1996). It binds in vivo to a GT motif with...

Latency, chromatin remodeling, and reactivation of human cytomegalovirus in the dendritic cells of healthy carriers

“…We included a control PCR on the samples targeting the HS4 region of the cellular ␤-globin locus control region to preclude the possibility that any negative immunoprecipitation was due to a nonspecific inability to immunoprecipitate chromatin. The HS4 region of the cellular ␤-globin promoter is acetylated in CD34 ϩ cells and becomes deacetylated͞methylated upon differentiation (29). Consistent with this analysis, we observed that the HS4 region was associated with acetylated histones in CD34 ϩ cells (Fig.…”

The binding of the ubiquitous transcription factor Sp1 at the locus control region represses the expression of β-like globin genes

“…Hence, the binding of Sp1 at the ␥-globin promoter could still be seen but was totally lost at the ␤-globin promoter after induction. Sp1 and EKLF binding at the ␤-LCR can repress the spreading of heterochromatin in erythroid cells (31). HS5 has been reported as an insulator in human ␤-globin locus (32), and the binding of Sp1 at HS5 after induction is in agreement with these findings.…”

Subcellular Transport of EKLF and Switch-On of Murine Adult βmaj Globin Gene Transcription

“…Evidence has been presented that EKLF is functional in the primitive erythroid lineage, but its activity with respect to turning on the adult ␤ globin gene is greatly suppressed in the early embryonic erythroid cells for unknown reasons (see references 3 and 54 and references therein). Also, overexpression of EKLF leads to increased transcription of the adult ␤ globin gene and an earlier switch from fetal ␥ to adult ␤ globin expression (33,53), while decreasing the concentration of EKLF results in lowered expression of ␤ with a reciprocal increase in ␥ gene expression (58).…”