In vivo protein-DNA interactions at the beta-globin gene locus.

We have investigated in vivo protein-DNA interactions in the beta-globin gene locus by dimethyl sulfate (DMS) footprinting in K562 cells, which express epsilon- and gamma-globin but not beta-globin. In the locus control region, hypersensitive site 2 (HS-2) exhibited footprints in several putative protein binding motifs. HS-3 was not footprinted. The beta promoter was also not footprinted, while extensive footprints were observed in the promoter of the active gamma-globin gene. No footprints were seen in the A gamma and beta 3' enhancers. With several motifs, additional protein interactions and alterations in binding patterns occurred with hemin induction. In HeLa cells, some footprints were observed in some of the motifs in HS-2, compatible with the finding that HS-2 has some enhancer function in HeLa cells, albeit much weaker than its activity in K562 cells. No footprint was seen in B lymphocytes. In vivo footprinting is a useful method for studying relevant protein-DNA interactions in erythroid cells.     

A human beta-globin gene fused to the human beta-globin locus control region is expressed at high levels in erythroid cells of mice engrafted with retrovirus-transduced hematopoietic stem cells

Retroviral-mediated gene transfer of human beta-globin provides a model system for the development of somatic gene therapy for hemoglobinopathies. Previous work has shown that mice receiving a transplant of bone marrow cells infected with a retroviral vector containing the human beta-globin gene can express human beta-globin specifically in erythroid cells; however, the level of expression of the transduced globin gene was low (1% to 2% per gene copy as compared with that of the endogenous mouse beta-globin gene). We report here the construction of a recombinant retrovirus vector encoding a human beta- globin gene fused to the 4 major regulatory elements of the human beta- globin locus control region (LCR). The LCR cassette increases the level of expression of the globin gene in murine erythroleukemia cells by 10- fold. To study the level of expression in vivo, mouse bone marrow cells were infected with virus-producing cells and the transduced cells were injected into lethally irradiated recipients. In the majority of provirus-containing mice (up to 75%), expression of human beta-globin in peripheral blood was detected at least 3 to 6 months after transplantation. Twelve animals representative of the level of expression of the transduced gene in blood (0.04% to 3.2% of the endogenous mouse beta-globin RNA) were selected for further analysis. A range of 0.4% to 12% of circulating erythrocytes stained positive for human beta-globin protein. Based on these values, the level of expression of the transduced gene per cell was estimated to be 10% to 39% of the endogenous mouse beta-globin gene. These data demonstrate that fusion of the LCR to the beta-globin gene in a retroviral vector increases the level of beta-globin expression in murine erythroleukemia cells and suggest that high-level expression can be obtained in erythroid cells in vivo after transduction into hematopoietic stem cells.     

Dissection of the enhancer activity of beta-globin 5' DNase I-hypersensitive site 2 in transgenic mice.

The beta-globin locus control region (LCR) consists of four erythroid-specific DNase I-hypersensitive sites, which are necessary for high-level expression of the beta-like globin genes in erythroid tissues. One of these sites, designated 5'HS-2, functions as an erythroid-specific enhancer element in transfection and transgenic mouse experiments. Recent transfection experiments and studies of DNA-protein interactions have localized the 5'HS-2 enhancer to 18 nucleotides that contain a binding site for both the erythroid-specific factor nuclear factor erythroid 2 (NFE-2) and for activator protein 1 (AP-1). To define the sequences necessary for in vivo enhancer activity, several deletion mutants of 5'HS-2 were linked to the human beta-globin gene and their activity was tested in transgenic mice. Three upstream fragments of 5'HS-2 [341, 374, and 412 base pairs (bp)], each of which contained the NFE-2/AP-1 sequences, resulted in beta-globin expression at levels equivalent to or higher than those observed with the entire 732-bp 5'HS-2 fragment. In contrast, a 358-bp downstream portion of 5'HS-2, which lacked the NFE-2/AP-1 sequences, resulted in beta-globin expression at the low levels seen with the beta-globin gene alone. Removal of the NFE-2/AP-1 sequences by a 67-bp internal deletion resulted in similar low levels of beta-globin expression. A 100-bp 5' fragment that contained the NFE-2/AP-1 sequences resulted in beta-globin expression that was higher than the beta-globin gene alone but lower than the entire 5'HS-2 fragment or the three larger upstream fragments. These studies demonstrate that the NFE-2/AP-1 sequences are essential for enhancer activity of 5'HS-2 but that other sequences are required for full activity in vivo.     

Properties of the mouse α-globin HS-26: Relationship to HS-40, the major enhancer of human α-globin gene expression

HS-26, the mouse homologue of HS-40, is the major regulatory element of the mouse α-globin gene locus. Like HS-40, HS-26 is located within an intron of a house-keeping gene; comparison of the nucleotide sequences of HS-26 and HS-40 reveals conservation of the sequences and positions of several DNA binding motifs in the 5′ regions of both elements (3 GATA, 2 NFE-2, and 1 CACCC sites) and the absence in HS-26 of three CACCC sites and one GATA site that are present in the 3′ region of HS-40, suggesting that the two elements might not be identical. We report here that when HS-26 is linked to a 1.5 kb Psti human α-globin gene fragment, it has a weak enhancer activity in induced MEL cells and in transgenic embryos, and it does not have any detectable activity in adult transgenic mice. This suggests that HS-26 does not have Locus Control Region (LCR) activity but can act as an enhancer during the embryonic life when integrated at a permissive locus. To further test the importance of HS-26 at its natural locus, we have generated embryonic stem cells and chimeric animals in which 350 bp containing HS-26 have been replaced by a neomycin resistance gene by homologous recombination. The sizes of the chimeras' red cells were then estimated by measuring forward scattering on a FacsScan apparatus in hypotonic conditions. This revealed that a fraction of the chimeric animals' red cells were smaller than normal mouse red cells and were very similar to cells from mice heterozygous for α-thalassemia. Density gradient analysis also suggested the presence of thalassemic cells. These results indicated that despite its lack of LCR activity, HS-26 is important for the regulation of the mouse α-globin gene locus. Am. J. Hematol. 54:30–39, 1997 © 1997 Wiley-Liss, Inc.     

Protein-DNA interactions in vivo of an erythroid-specific, human beta-globin locus enhancer.

The 5' DNase I-hypersensitive site 2 (5' HS-2) is an erythroid-specific enhancer located 11 kilobases (kb) upstream of the human beta-globin gene cluster. Presence in cis of 5' HS-2 confers a high level of erythroid cell-specific and developmentally regulated promoter activities of human globin genes in transfected cell cultures and in transgenic mice. Combining the use of the methylation protection assay and polymerase chain reaction, we have studied nuclear factor-DNA interactions of the 5' HS-2 enhancer in vitro and in vivo. The data from analyses of three different sequence motifs within 5' HS-2 represent three different modes of protein-DNA interaction with respect to cell-type specificities and in vivo vs. in vitro differences. First, a GATA-1 motif was found to bind nuclear factor(s), presumably the GATA-1 factor, present in K-562 cell extracts and in living K-562 cells. No such binding was seen in nonerythroid HeLa cells or extract. A second motif, NF-E2/AP1 (nuclear factor-erythroid 2/activator protein 1), consists of tandemly arranged dimers of AP1 binding consensus. The presence of either HeLa extract or K-562 extract protects the NF-E2/AP1 motif from methylation, but the footprints are different. This is most likely due to different protein-DNA contacts of the AP1-DNA complex formed in HeLa extract and the NF-E2-DNA complex in K-562 extract. In vivo methylation protection patterns of this motif parallel those observed in vitro, suggesting that it is also bound by NF-E2 in K-562 cells and by AP1 in HeLa cells. Finally, a GT-I motif binds apparently to one or more similar factors in both types of nuclear extracts, but the in vivo methylation protection patterns are not identical between living HeLa and K-562 cells. These data provide direct evidence that specific nuclear factor-DNA complexes form in vivo at functionally important sequence motifs of the 5' HS-2 enhancer in erythroid cells. The detection of conformationally different nuclear factor-DNA complexes at the same sequence motifs in HeLa and Raji cell lines also raises interesting questions regarding the origin and function of these complexes in nonerythroid cells.     

An erythrocyte-specific DNA-binding factor recognizes a regulatory sequence common to all chicken globin genes.

We have identified a protein present only in erythroid cells that binds to two adjacent sites within an enhancer region of the chicken beta-globin locus. Mutation of the sites, so that binding by the factor can no longer be detected in vitro, leads to a loss of enhancing ability, assayed by transient expression in primary erythrocytes. Binding sites for the erythroid-specific factor (Eryf1) are found within regulatory regions for all chicken globin genes. A strong Eryf1 binding site is also present within the enhancer of at least one human globin gene, and proteins from human erythroid cells (but not HeLa cells) bind to both the chicken and the human sites.     

The β-globin promoter is important for recruitment of erythroid Krüppel-like factor to the locus control region in erythroid cells

Erythroid Krüppel-like factor (EKLF), which binds to the CACCC box in the beta-globin promoter, is required for the expression of the beta-globin gene in adult erythroid cells. It was recently demonstrated that EKLF is also required for the activity of the beta-globin locus control region (LCR) 5'HS3. Some evidence suggests that the LCR and the beta-globin promoter interact in adult erythroid cells, and the network of protein-protein interactions that exists between these two elements may regulate how EKLF is recruited to the LCR. In this report, we use the PIN*POINT assay to study the role of the promoter on the recruitment of EKLF to 5'HS2 and 5'HS3 of the LCR. We find that recruitment of EKLF to 5'HS2 requires the TATA box, but recruitment to 5'HS3 depends on the CACCC and TATA boxes of the beta-globin promoter. Furthermore, recruitment of EKLF to 5'HS3 only occurred in beta-globin-expressing murine erythroid leukemia cells, whereas recruitment of EKLF to 5'HS2 occurred in both gamma-globin-expressing K562 cells and murine erythroid leukemia cells. Unlike EKLF, Sp1, which also binds to CACCC boxes, is not recruited to 5'HS3. We have also examined how one 5'HS affects the recruitment of EKLF to another 5'HS. We have found that the recruitment of EKLF to 5'HS3 depends on the presence of 5'HS2 in cis, but the recruitment to 5'HS2 does not depend on 5'HS3. Based on these results, we present a model that illustrates how EKLF may be recruited to the beta-globin locus.