Conserved elements containing NF-E2 and tandem GATA binding sites are required for erythroid-specific chromatin structure reorganization within the human beta-globin locus control region.

Proper expression of the genes of the human beta-globin gene locus requires the associated locus control region (LCR). Structurally, the LCR is defined by the presence of four domains of erythroid-specific chromatin structure. These domains, which have been characterized as DNase I hypersensitive sites (HSs), comprise the active elements of the LCR. The major focus of this research is to define the cis -acting elements which are required for the formation of these domains of unique chromatin structure. Our previous investigations on the formation of LCR HS4 demonstrated that NF-E2 and tandem, inverted GATA binding sites are required for the formation of the native HS. Similarly arranged NF-E2 and tandem GATA sites are present within the core regions of the other human LCR HSs and are evolutionarily conserved. Using site-directed mutagenesis of human HSs 2 and 3 we have tested the hypothesis that these NF-E2 and GATA sites are common requirements for the formation of all LCR HSs. We find that mutation of these elements, and particularly the GATA elements, results in a decrease or complete loss of DNase I hypersensitivity. These data imply the presence of common structural elements within the core of each LCR HS which are required for erythroid-specific chromatin structure reorganization.     

Hypersensitive site 4 of the human beta globin locus control region.

The Locus Control Region (LCR) of the human beta globin gene domain is defined by four erythroid-specific DNasel hypersensitive sites (HSS) located upstream of this multigene cluster. The LCR confers copy number dependent high levels of erythroid specific expression to a linked transgene, independent of the site of integration. To assess the role of the individual hypersensitive sites of the LCR, we have localized HSS4 to a 280bp fragment that is functional both in murine erythroleukaemia (MEL) cells and in transgenic mice. This fragment coincides with the major area of hypersensitivity 'in vivo' and contains a number of DNasel footprints. Bandshift analysis shows that these footprints correspond to binding sites for the erythroid specific proteins GATA1 and NF-E2 and a number of ubiquitous proteins, including jun/fos, Sp1 and TEF2.     

A minimal ankyrin promoter linked to a human gamma-globin gene demonstrates erythroid specific copy number dependent expression with minimal position or enhancer dependence in transgenic mice

In red blood cells ankyrin (ANK-1) provides the primary linkage between the erythrocyte membrane skeleton and the plasma membrane. We have previously demonstrated that a 271-bp 5'-flanking region of the ANK-1 gene has promoter activity in erythroid, but not non-erythroid, cell lines. To determine whether the ankyrin promoter could direct erythroid-specific expression in vivo, we analyzed transgenic mice containing the ankyrin promoter fused to the human (A)gamma-globin gene. Sixteen of 17 lines expressed the transgene in erythroid cells indicating nearly position-independent expression. We also observed a significant correlation between the level of Ank/(A)gamma-globin mRNA and transgene copy number. The level of Ank/(A)gamma mRNA averaged 11% of mouse alpha-globin mRNA per gene copy at all developmental stages. The addition of the HS2 enhancer from the beta-globin locus control region to the Ank/(A)gamma-globin transgene resulted in Ank/(A)gamma-globin mRNA expression in embryonic and fetal erythroid cells in six of eight lines but resulted in absent or dramatically reduced levels of Ank/(A)gamma-globin mRNA in adult erythroid cells in eight of eight transgenic lines. These data indicate that the minimal ankyrin promoter contains all sequences necessary and sufficient for erythroid-specific, copy number-dependent, position-independent expression of the human (A)gamma-globin gene.     

Position independence and proper developmental control of gamma-globin gene expression require both a 5'' locus control region and a downstream sequence element.

We have analyzed the expression of human gamma-globin genes during development in F2 progeny of transgenic mice carrying two types of constructs. In the first type, gamma-globin genes were linked individually to large (approximately 4-kb) sequence fragments spanning locus control region (LCR) hypersensitive site 2 (HS2) or HS3. These LCR fragments contained not only the core HS elements but also extensive evolutionarily conserved flanking sequences. The second type of construct contained tandem gamma- and beta-globin genes linked to identical HS2 or HS3 fragments. We show that gamma-globin expression in transgenic mice carrying HS2 gamma or HS3 gamma constructs is highly sensitive to position effects and that such effects override the cis regulatory elements present in these constructs to produce markedly different developmental patterns of gamma-globin expression in lines carrying the same transgene. In contrast, gamma-globin expression in both HS2 gamma beta and HS3 gamma beta mice is sheltered from position effects and the developmental patterns of gamma-globin expression in lines carrying the same transgene are identical and display stage-specific regulation. The results suggest that cis regulatory sequences required for proper developmental control of fetal globin expression in the presence of an LCR element reside downstream from the gamma genes.     

Position effects in mice carrying a lacZ transgene in cis with the beta-globin LCR can be explained by a graded model.

We studied transgenic mice carrying the lacZ reporter gene linked to the erythroid-specific beta-globin promoter and beta-globin locus control region (LCR). Previously, we had demonstrated that the total level of expression of beta-galactosidase enzyme, which is the product of the lacZ gene, varies widely between different transgenic mice due to position effects at the sites of transgene integration. Here, using the X-gal based in situ assay for beta-galactosidase activity, we found that the percent erythroid cells that expressed the transgene also varied widely between the mice. Moreover, a kinetic analysis showed that the average beta-galactosidase content per expressing cell varied both between samples of different transgenic descent and between erythroid cells within each sample, demonstrating that the variable expression of this lacZ transgene was being controlled in a graded manner. These results suggest that the beta-globin LCR enhancers function through a graded model, which is described, rather than the binary mechanism that has been proposed previously for other enhancers.     

Function of the upstream hypersensitive sites of the chicken beta-globin gene cluster in mice.

We have shown previously that the chicken beta A-globin gene, with its 3' enhancer, is expressed in a copy number-dependent manner in transgenic mice. The expression level was low but increased approximately 6-fold upon inclusion of 11 kb of upstream DNA containing four DNase I hypersensitive sites. To study the effect of the individual upstream hypersensitive sites on transgene expression, we produced lines of mice in which the individual upstream sites were linked to the beta A gene and enhancer. RNA levels were measured in blood from adult animals. With each of these four constructs, the level of transgene RNA per DNA copy varied over a > 20-fold range. These data suggest that addition of a hypersensitive site to the beta A-globin/enhancer region abrogates its position independent expression. The average beta A-globin expression per copy in the lines carrying an upstream site was comparable with that in lines without an upstream site. Thus, no single upstream hypersensitive site accounts for the higher level of beta A-globin expression seen in mice containing the complete upstream region. We had shown previously that control of the chicken beta-globin cluster is distributed between at least two regions, the beta A/epsilon enhancer and the upstream region. Our current results suggest that the control mediated by the upstream DNA is itself distributed and is not due to a single hypersensitive site.     

Human HMG box transcription factor HBP1: a role in hCD2 LCR function

The locus control region (LCR) of the human CD2 gene (hCD2) confers T cell-specific, copy-dependent and position-independent gene expression in transgenic mice. This LCR consists of a strong T cell-specific enhancer and an element without enhancer activity (designated HSS3), which is required for prevention of position effect variegation (PEV) in transgenic mice. Here, we identified the HMG box containing protein-1 (HBP1) as a factor binding to HSS3 of the hCD2 LCR. Within the LCR, HBP1 binds to a novel TTCATTCATTCA sequence that is higher in affinity than other recently reported HBP1-binding sites. Mice transgenic for a hCD2 LCR construct carrying a deletion of the HBP1-binding sequences show a propensity for PEV if the transgene integrates in a heterochromatic region of the chromosome such as the centromere or telomere. We propose that HBP1 plays an important role in chromatin opening and remodelling activities by binding to and bending the DNA, thus allowing DNA-protein and/or protein-protein interactions, which increase the probability of establishing an active locus.     

A dominant chromatin-opening activity in 5' hypersensitive site 3 of the human beta-globin locus control region.

Single-copy human beta-globin transgenes are very susceptible to suppression by position effects of surrounding closed chromatin. However, these position effects are overcome by a 20 kbp DNA fragment containing the locus control region (LCR). Here we show that the 6.5 kbp microlocus LCR cassette reproducibly directs full expression from independent single-copy beta-globin transgenes. By testing individual DNase I-hypersensitive sites (HS) present in the microlocus cassette, we demonstrate that the 1.5 kbp 5'HS2 enhancer fragment does not direct beta-globin expression from single-copy transgenes. In contrast, the 1.9 kbp 5'HS3 fragment directs beta-globin expression in five independent single-copy transgenic mouse lines. Moreover, the 5'HS3 core element and beta-globin proximal promoter sequences are DNase I hypersensitive in fetal liver nuclei of these expressing transgenic lines. Taken together, these results demonstrate that LCR activity is the culmination of at least two separable functions including: (i) a novel activity located in 5'HS3 that dominantly opens and remodels chromatin structure; and (ii) a recessive enhancer activity residing in 5'HS2. We postulate that the different elements of the LCR form a 'holocomplex' that interacts with the individual globin genes.     

Multiple elements in human beta-globin locus control region 5' HS 2 are involved in enhancer activity and position-independent, transgene expression.

The human beta-globin Locus Control Region (LCR) has two important activities. First, the LCR opens a 200 kb chromosomal domain containing the human epsilon-, gamma- and beta-globin genes and, secondly, these sequences function as a powerful enhancer of epsilon-, gamma- and beta-globin gene expression. Erythroid-specific, DNase I hypersensitive sites (HS) mark sequences that are critical for LCR activity. Previous experiments demonstrated that a 1.9 kb fragment containing the 5' HS 2 site confers position-independent expression in transgenic mice and enhances human beta-globin gene expression 100-fold. Further analysis of this region demonstrates that multiple sequences are required for maximal enhancer activity; deletion of SP1, NF-E2, GATA-1 or USF binding sites significantly decrease beta-globin gene expression. In contrast, no single site is required for position-independent transgene expression; all mice with site-specific mutations in 5' HS 2 express human beta-globin mRNA regardless of the site of transgene integration. Apparently, multiple combinations of protein binding sites in 5' HS 2 are sufficient to prevent chromosomal position effects that inhibit transgene expression.     

Synergistic and additive properties of the beta-globin locus control region (LCR) revealed by 5'HS3 deletion mutations: implication for LCR chromatin architecture

Deletion of the 234-bp core element of the DNase I hypersensitive site 3 (5'HS3) of the locus control region (LCR) in the context of a human beta-globin locus yeast artificial chromosome (beta-YAC) results in profound effects on globin gene expression in transgenic mice. In contrast, deletion of a 2.3-kb 5'HS3 region, which includes the 234-bp core sequence, has a much milder phenotype. Here we report the effects of these deletions on chromatin structure in the beta-globin locus of adult erythroblasts. The 234-bp 5'HS3 deletion abolished histone acetylation throughout the beta-globin locus; recruitment of RNA polymerase II (pol II) to the LCR and beta-globin gene promoter was reduced to a basal level; and formation of all the 5' DNase I hypersensitive sites of the LCR was disrupted. The 2.3-kb 5'HS3 deletion mildly reduced the level of histone acetylation but did not change the profile across the whole locus; the 5' DNase I hypersensitive sites of the LCR were formed, but to a lesser extent; and recruitment of pol II was reduced, but only marginally. These data support the hypothesis that the LCR forms a specific chromatin structure and acts as a single entity. Based on these results we elaborate on a model of LCR chromatin architecture which accommodates the distinct phenotypes of the 5'HS3 and HS3 core deletions.