Control of globin gene expression during development and erythroid differentiation

Abstract

Extensive studies during the last 30 years have led to considerable understanding of cellular and molecular control of hemoglobin switching. Cell biology studies in the 1970s defined the control of globin genes during erythroid differentiation and led to development of therapies for sickle cell disease. Molecular investigations of the last 20 years have delineated the two basic mechanisms that control globin gene activity during development--autonomous silencing and gene competition. Studies of hemoglobin switching have provided major insights on the control of gene loci by remote regulatory elements. Research in this field has an impact on understanding regulatory mechanisms in general and is of particular importance for eventual development of molecular cures for sickle cell disease and beta thalassemia.

Figure 1

Model of regulation of fetal hemoglobin production in adult individuals (see text). This model was instrumental for stimulating cell biology studies and experiments in primates [9,25,26] that led to the application of hydroxyurea treatment in sickle cell disease.

Figure 2

Diagram of the human β globin locus.

Figure 3

The competitive mechanism of hemoglobin switching. (A) This cartoon illustrates the initial experiments in transgenic mice that provided evidence in support of the competition mechanism of human fetal to adult globin gene switching [65]. Developmental control is lost when the human β or γ globin genes are linked individually to the LCR. However, control is fully restored when both the β and the γ genes in their normal chromosomal arrangement are linked to this dominant regulatory element; the γ gene is expressed only in the fetal stage and the β gene only in the adult stage. The best interpretation of these results is gene competition. As shown in (B), fetal and adult globin genes compete for the interaction of the LCR. The transcriptional environment favors the interaction between the LCR and the γ gene in the fetal stage, resulting in silencing of β gene expression. The opposite occurs in the adult stage. Plenty of experimental evidence indicates that gene competition is the mechanism of silencing of the β gene in the embryonic and fetal erythropoiesis.

Figure 4

The autonomous silencing mechanism of hemoglobin switching. All sequences required for silencing are included in proximal to the gene regulatory elements. Silencing is autonomous, no competition by another globin gene is involved. The concept is illustrated by the autonomous silencing of the ε globin gene. Transcriptional factors bind on silencing sequences located in the proximal and the distal ε gene promoter and form a repressor complex that disrupts the interaction between the gene and the LCR, resulting in turning off of ε gene expression.

Figure 5

Experimental evidence for the existence of γ gene-specific fetal transcriptional factor has been obtained from cell fusion experiments. (A) Results of transfer of lymphoid origin human chromosome 11 into adult or fetal-like murine erythroleukemic cells [80]. The silent human γ globin genes are activated in the fetal-like environment. (B) Results of transfer of a human β locus YAC into the adult or fetal-like murine erythroleukemia cells [81]. The γ globin gene of the β YAC fail to switch in the fetal environment.

Figure 6

Sequences in the γ gene promoter as well as sequences of the LCR are involved in the activation of the γ globin gene. The diagrams illustrate the results of two experiments using β locus YAC transgenic mice. (A) When the core element of DNAse I hypersensitive site 3 (HS3) of the LCR is deleted, γ gene expression in fetal erythropoiesis is essentially abolished [50], providing evidence that this core sequence contains elements involved in γ gene activation. (B) Even mutations of a single motif of the core sequence of HS3 result in striking decrease of γ gene expression [83]. Experiments using β locus YAC transgenic mice have substantially increased our knowledge of the cis regulatory elements involved in globin gene switching.