Locus control region mediated regulation of adult beta-globin gene expression

Abstract

Many genes residing in gene clusters and expressed in a differentiation or developmental-stage specific manner are regulated by locus control regions (LCRs). These complex genetic regulatory elements are often composed of several DNAse I hypersensitive sites (HS sites) that function together to regulate the expression of several cis-linked genes. Particularly well characterized is the LCR associated with the beta-globin gene locus. The beta-globin LCR consists of five HS sites that are located upstream of the beta-like globin genes. Recent data demonstrate that the LCR is required for the association of the beta-globin gene locus with transcription foci or factories. The observation that RNA polymerase II associates with the LCR in erythroid progenitor or hematopoietic stem cells which do not express the globin genes suggests that the LCR is always in an accessible chromatin configuration during differentiation of erythroid cells. We propose that erythroid specific factors together with ubiquitous proteins mediate a change in chromatin configuration that juxtaposes the globin genes and the LCR. The proximity then facilitates the transfer of activities from the LCR to the globin genes. In this article we will discuss recent observations regarding beta-globin locus activation with a particular emphasis on LCR mediated activation of adult beta-globin gene expression.

Figure 1. Summary of proteins and co-regulators interacting with LCR element HS2 and with the adult β-globin gene promoter

Shown on top is the overall organization of the human β-globin gene locus depicting the LCR and the embryonic (ε), fetal (Gγ and Aγ), and adult (δ and β) globin genes. The diagram below illustrates the overall organization of regulatory elements associated with adult β-globin gene regulation. Transcription factor binding sites (boxes) in LCR element HS2 (on the left) and the β-globin promoter (on the right) are bound by transcription factors (circles) that either activate (shown below the binding sites) or repress (shown above the binding sites) transcription through the recruitment of co-regulators (triangles). The binding sequences, interacting proteins, and co-regulators are listed in the table. DNA binding motifs as well as interacting proteins and their known co-regulators are color matched. No co-regulators have been described for Bach1 and Bp1 (indicated by question marks).

Figure 2. LCR holocomplex and transfer of elongation competent transcription complexes to the adult β-globin gene promoter

In progenitor cells the HS sites interact with each other, which is mediated by protein/DNA and protein/protein interactions. This involves, among other proteins, Bach1, Sp1, and USF. Transcription complexes are already recruited to the LCR and may transcribe through the HS sites to maintain accessible domains during differentiation of erythroid cells. In mature erythroid cells expressing the adult β-globin gene LCR HS sites are bound by erythroid-specific and ubiquitously expressed proteins and adopt a more open and dynamic configuration. Elongation incompetent transcription complexes are attracted and loosely associate with proteins or co-regulators bound at the LCR. These transcription complexes are transferred to the β-globin gene promoter, which contains strong basal promoter elements. Once recruited to the promoter, LCR associated activities stabilize the polymerase, protect it from proteasome mediated degradation and/or provide CTD kinase activity.

Figure 3. LCR mediated regulation of β-globin gene expression

The LCR adopts or retains an accessible chromatin conformation in hematopoietic stem cells. As discussed in figure 2 transcription complexes are recruited and maintain an open configuration at this stage. The rest of the globin locus is in an inaccessible configuration. The entire globin locus is packed into heterochromatin when the cells differentiate into non-erythroid cells. During differentiation of erythroid cells erythroid-specific factors and associated co-regulators open sub-domains in the globin gene locus. These sub-domains unfold and localize to an LCR proximal position. Erythroid-specific and ubiquitously active proteins mediate the transfer of transcription complexes from the LCR to the globin gene promoter, stabilize the transcription complex, and enhance transcription elongation.