Autonomous silencing as well as competition controls gamma-globin gene expression during development.

To investigate the control of the gamma-globin gene during development, we produced transgenic mice in which sequences of the beta-gene promoter were replaced by equivalent sequences of the gamma-gene promoter in the context of a human beta-globin locus yeast artificial chromosome (betaYAC) and analyzed the effects on globin gene expression during development. Replacement of 1,077 nucleotides (nt) of the beta-gene promoter by 1,359 nt of the gamma promoter resulted in striking inhibition of the gamma-promoter/beta-gene expression in the adult stage of development, providing direct evidence that the expression of the gamma gene in the adult is mainly controlled by autonomous silencing. Measurements of the expression of the gamma promoter/beta-globin gene as well as the wild gamma genes showed that gene competition is also involved in the control of gamma-gene expression in the fetal stage of development. We conclude that autonomous silencing is the main mechanism controlling gamma-gene expression in the adult, while autonomous silencing as well as competition between gamma and beta genes contributes to the control of gamma to beta switching during fetal development.

Transcriptional potential of the gamma-globin gene is dependent on the CACCC box in a developmental stage-specific manner.

To test the role of CACCC box on gamma-globin gene activation, the CACCC box was deleted or mutated and gamma-gene expression was monitored in transgenic mice. Disruption of the CACCC box had no effect on gamma-gene expression in the cells of embryonic erythropoiesis but it strikingly reduced gamma-gene expression in fetal erythropoiesis, and abolished gamma-gene expression in adult erythroid cells. The CACCC mutation diminished HS formation, as well as TBP and polII recruitment at the gamma-gene promoter; however, it only resulted in slight or no effects on histone H3 and H4 acetylation in adult erythropoiesis. Our findings indicate that each basic cis element of the proximal gamma-gene promoter, i.e. CACCC, CCAAT or TATA box, can be disrupted without affecting the activation of gamma gene in embryonic erythroid cells. We propose that the trans factors recruited by the three boxes interact with each other to form a 'promoter complex'. In embryonic erythropoiesis the locus control region enhancer is able to interact with the complex even when components normally binding to one of the motifs are missing, but it can only activate an intact 'promoter complex' in adult erythroid cells.

Evidence that DNase I hypersensitive site 5 of the human beta-globin locus control region functions as a chromosomal insulator in transgenic mice.

We have previously reported that DNase I hypersensitive site 5 (5'HS5) of the human beta-globin locus control region functions as a chromatin insulator in stable transfection assays. In this report we show that a 3.2 kb DNA fragment containing the entire 5'HS5 region can protect a position-sensitive (A)gamma-globin gene against position effects in transgenic mice. Bracketing is required for function of 5'HS5 as an insulator. The 5'HS5 insulator operates in adult as well as in embryonic murine erythroid cells. The insulator has no significant stimulatory effects of its own. These results indicate that 5'HS5 can function as a chromatin insulator in vivo.

Transcriptional environment and chromatin architecture interplay dictates globin expression patterns of heterospecific hybrids derived from undifferentiated human embryonic stem cells or from their erythroid progeny.

To explore the response of ß globin locus with established chromatin domains upon their exposure to new transcriptional environments, we transferred the chromatin-packaged ß globin locus of undifferentiated human embryonic stem cells (hESCs) or hESC-derived erythroblasts into an adult transcriptional environment. Distinct globin expression patterns were observed. In hESC-derived erythroblasts where both ε and γ globin were active and marked by similar chromatin modifications, ε globin was immediately silenced upon transfer, whereas γ globin continued to be expressed for months, implying that different transcriptional environments were required for their continuing expression. Whereas ß globin was silent both in hESCs and in hESC-derived erythroblasts, ß globin was only activated upon transfer from hESCs, but not in the presence of dominant γ globin transferred from hESC-derived erythroblasts, confirming the competing nature of γ versus ß globin expression. With time, however, silencing of γ globin occurred in the adult transcriptional environment with concurrent activation of ß-globin, accompanied by a drastic change in the epigenetic landscape of γ and ß globin gene regions without apparent changes in the transcriptional environment. This switching process could be manipulated by overexpression or downregulation of certain transcription factors. Our studies provide important insights into the interplay between the transcription environment and existing chromatin domains, and we offer an experimental system to study the time-dependent human globin switching.

The higher structure of chromatin in the LCR of the beta-globin locus changes during development.

The beta-globin locus control region (LCR) is able to enhance the expression of all globin genes throughout the course of development. However, the chromatin structure of the LCR at the different developmental stages is not well defined. We report DNase I and micrococcal nuclease hypersensitivity, chromatin immunoprecipitation analyses for histones H2A, H2B, H3, and H4, and 3C (chromatin conformation capture) assays of the normal and mutant beta-globin loci, which demonstrate that nucleosomes at the DNase I hypersensitive sites of the LCR could be either depleted or retained depending on the stages of development. Furthermore, MNase sensitivity and 3C assays suggest that the LCR chromatin is more open in embryonic erythroblasts than in definitive erythroblasts at the primary- and secondary-structure levels; however, the LCR chromatin is packaged more tightly in embryonic erythroblasts than in definitive erythroblasts at the tertiary chromatin level. Our study provides the first evidence that the occupancy of nucleosomes at a DNase I hypersensitive site is a developmental stage-related event and that embryonic and adult cells possess distinct chromatin structures of the LCR.

Differences of globin transgene expression in stably transfected cell lines and transgenic mice.

Previous studies demonstrated that DNase I hypersensitive site -40 (HS-40) of the alpha-globin locus is capable of greatly enhancing expression of a hybrid beta/gamma-globin transcriptional unit in plasmid-transfected murine erythroleukemia (MEL) cells. However, as reported here, this same gamma-globin gene expression cassette was only transcribed at trace amounts in erythroid cells of transgenic mice. This lack of expression was not directly attributable to the beta/gamma-globin transcriptional unit, since this same unit linked to a composite beta-globin locus control region was expressed at high levels in transgenic mice. This lack of expression was also not directly attributable to chromosomal position effects, since addition of chromatin insulators failed to increase the frequency of expression. DNase I hypersensitivity and chromatin immunoprecipitation assays demonstrated that the lack of expression was correlated with a closed chromatin structure. We hypothesize that transgenes undergo dynamic changes in chromatin conformation following chromosomal integration and that the discrepant results reported here can be attributed to the relatively high level of chromatin remodeling that occurs in the transgenic mouse model, coupled with the relative inability of the HS-40 element to maintain an open chromatin state under such conditions.

Juxtaposition of the HPFH2 enhancer is not sufficient to reactivate the gamma-globin gene in adult erythropoiesis.

Previous studies have suggested that juxtaposition of a downstream enhancer to the fetal gamma-globin gene results in reactivation of the gamma-gene in adult erythrocytes of individuals with hereditary persistence of fetal hemoglobin (HPFH). To test the hypothesis in a much stricter basis, we produced beta locus YAC transgenic mice carrying an exact beta locus replicate of a deletional HPFH mutation, HPFH 2. Although the gamma-globin gene was expressed in the HPFH 2/beta locus YAC (HPFH2/YAC) transgenic mice in the early stage of development, it was completely silenced in the adult mice. The failure of gamma-gene reactivation by the juxtaposed HPFH2 enhancer contradicts the results of previous studies. We speculate that the discrepant results reflect differences in the distance between the locus of region (LCR) and the gamma-globin gene characteristic of the plasmid, cosmid or YAC constructs used for production of transgenic mice. The difference in the phenotype of the HPFH2/YAC transgenic mice and the humans with HPFH2 mutation suggests that in addition to juxtaposition of HPFH enhancers, the upstream region that is absent in the beta-YAC construct might be involved in gamma-gene reactivation in HPFH individuals. The DNase I hypersensitive sites of the LCR were well formed and the chromatin histones were acetylated. A moderate level of pol II binding was detected in the LCR, despite the fact that no transcription occurred in the globin-genes of the adult HPFH2/YAC transgenic mice. The results suggest that formation of the LCR chromatin structure in erythroid cells is independent of globin-gene transcription in the locus.

The minimal promoter plays a major role in silencing of the galago gamma-globin gene in adult erythropoiesis.

The human gamma-globin gene and its orthologous galago gamma-globin gene evolved from an ancestral epsilon-globin gene. In galago, expression of the gamma-gene remained restricted to the embryonic stage of development, whereas in humans, expression of the gamma-gene was recruited to the fetal stage. To localize the cis-elements responsible for this developmentally distinct regulation, we studied the expression patterns of the human gamma-gene driven by either the human or the galago gamma-promoters in transgenic mice. gamma-gene transcription driven by either promoter reached similar levels in embryonic erythropoiesis. In adult erythropoiesis the gamma-gene was silenced when controlled by the galago gamma-promoter, but it was expressed at a high level when it was linked to the human gamma-promoter. By a series of gamma-promoter truncations the sequences required for the down-regulation of the galago gamma-globin gene were localized to the minimal promoter. Furthermore, by interchanging the TATA, CCAAT, and CACCC elements between the human and galago minimal promoters we found that whereas each box made a developmentally distinctive contribution to gamma-globin gene expression, the CACCC box was largely responsible for the down-regulation of the gamma-gene in adult erythropoiesis.

Developmentally specific role of the CCAAT box in regulation of human gamma-globin gene expression.

The CCAAT box is a widespread motif in eukaryotic promoters. In this study we demonstrate that the effects of the CCAAT box on gamma-globin gene activation are developmentally distinct. Although this promoter element is essential for high level gamma gene expression in adult erythropoiesis, it plays little role in embryonic erythroid cells. The CCAAT mutation in the human gamma-globin gene promoter impairs recruitment of TATA-binding protein (TBP), TFIIB, and RNA polymerase II in adult splenic erythroblasts but not in embryonic erythroid cells. We also show that the efficiency of gamma gene transcription is correlated with recruitment of TBP on the TATA box but that the level of TBP recruitment is not nuclear factor Y (NF-Y)-dependent. Our data also suggest that it is unlikely that transcriptional stimulation by the CCAAT box is exerted through direct protein-protein interaction between NF-Y and TBP.

Transcriptional potentials of the beta-like globin genes at different developmental stages in transgenic mice and hemoglobin switching.

Developmental-stage-specific regulation and physiological levels of expression of the globin genes can be recaptured in transgenic mice carrying a YAC/BAC- or cosmid-based construct. By contrast, proper developmental regulation and high-level expression cannot be achieved coordinately in transgenic mice carrying a more manipulated construct, such as a plasmid-based globin gene construct. These differences provide us an opportunity to define the requirements for a developmentally regulated, high-level expression of the globin genes in vivo. To achieve this, as a first step, we studied maximum transcriptional potentials of the beta-globin genes at various stages of development. microLCR-enhanced expression of the epsilon-, gamma-, and beta-globin genes driven by their minimal promoters was estimated and compared with that in betaYAC transgenic mice. Quantitative measurements of steady state mRNA levels of the epsilon-, gamma-, and beta-globin genes showed that the microLCR was able to enhance expression of each beta-like globin gene to levels similar to those in the betaYAC mice. Moreover, transcriptional potentials of each globin gene were unchanged during the entire course of development. These observations indicate that the highest level of expression of the globin genes can be achieved in both embryonic and definitive erythropoiesis regardless of developmental specificity of the genes. This finding implies that transcription suppression is the major mechanism of the developmental specificity of the expression of the beta-like globin genes.

Developmental specificity of recruitment of TBP to the TATA box of the human gamma-globin gene.

It is unclear whether the core promoter is involved in developmental regulation. To address this question, we mutated the TATA box of the human gamma-globin gene, produced transgenic mice, and examined the effect of the mutation during the course of mouse development. In our test system, the gamma-globin gene is expressed at similar levels in the embryonic and adult erythroid cells. The TATA box mutation dramatically reduced expression of the gamma-globin gene in the adult but not in embryonic erythroid cells. In addition, the disruption of the gamma TATA box significantly reduced the recruitment of TATA box-binding protein (TBP) in the adult cells, but not in embryonic cells, suggesting that the recruitment of TBP to the gamma gene promoter is developmentally specific. Similarly, the recruitment of transcription factor II B and RNA polymerase II to the gamma promoter was affected in the adult but not in embryonic cells. The distinct effects of the TATA mutation in the embryonic and adult developmental stages suggest that the basal transcription apparatus can be recruited to a core promoter in a developmental stage-dependent manner. The TATA mutation resulted in a shift of transcription initiation site 6 bp or longer upstream to the cap site both in the embryonic and adult erythrocytes. We conclude that the TATA box determines the initiation site but not the efficiency of transcription of the gamma-globin gene.