Upstream G gamma-globin and downstream beta-globin sequences required for stage-specific expression in transgenic mice

Developmental changes in DNA methylation and covalent histone modifications of chromatin associated with the ε-, γ-, and β-globin gene promoters in Papio anubis

The baboon is a suitable and relevant animal model to study the mechanism of human globin gene switching. This investigation addresses the role of DNA methylation and histone coding in globin gene switching in the baboon, Papio anubis. Bisulfite sequencing and chromatin immunoprecipitation studies were performed in erythroid cells purified from fetuses of varying gestational ages and from adult bone marrow to analyze the manner that changes in DNA methylation of the epsilon-, gamma-, and beta-globin promoters and association of ac-H3, ac-H4, H3-dimeK4, H3-dimeK36, and H3-dimeK79 with the epsilon-, gamma-, and beta-globin promoters occur during development. Changes in DNA methylation of the epsilon- and gamma-globin gene promoters during transitional stages of globin gene switching were consistent with the stochastic model of methylation and a role of DNA methylation in gene silencing. Enrichment of ac-H3, ac-H4, and pol II at the promoters of developmentally active genes was observed, while the pattern of distribution of H3-dimeK4 and H3-dimeK79 suggests that these modifications are found near both currently and formerly active promoters. Enrichment of H3-dimeK36 at the silenced epsilon-globin gene promoter was observed. These studies demonstrate that coordinated epigenetic modifications in the chromatin structure of the beta-like globin gene promoters accompany the highly regulated changes in expression patterns of these genes during development.

In Vivo Silencing of the Human γ-Globin Gene in Murine Erythroid Cells Following Retroviral Transduction

Increased expression of fetal hemoglobin can ameliorate the clinical severity of sickle cell disease. Whereas temporary induction of fetal hemoglobin can be achieved by pharmacologic therapy, gene transfer resulting in high-level expression of the fetal gamma-globin gene may provide a permanent cure for sickle cell disease. We had previously developed a high-titer, genetically stable retroviral vector in which the human gamma-globin gene was linked to HS-40, the major regulatory element of the human alpha-globin gene cluster. Based on experience in transgenic mice, the truncated promoter of the gamma-globin gene of this vector should be active in adult erythroid cells. Our earlier studies demonstrated that this retroviral vector can give rise to high-level expression of the human gamma-globin gene in murine erythroleukemia (MEL) cells. We have now utilized this vector to transduce murine bone marrow cells that were transplanted into W/W(v) recipient mice. Analysis of transduction of murine BFU-e's in vitro and peripheral blood cells from transplanted mice in vivo demonstrated efficient transfer of the human gamma-globin gene. However, in contrast to the high level of expression of the human gamma-globin gene of this vector in MEL cells, the gene was completely silent in vivo in all transplanted mice. These observations confirm that all the necessary regulatory elements responsible for the developmental stage-specific expression of the human gamma-globin gene reside in its proximal sequences. They also emphasize the differences between gene regulation in MEL cells, transgenic mice, and retroviral gene transfer vectors. For this form of globin gene therapy to succeed, the proximal regulatory elements of the human gamma-globin gene may have to be replaced with different regulatory elements that allow the expression of the gamma-globin coding sequences in adult red cells in vivo.

Genetic correction of sickle cell disease: insights using transgenic mouse models

Sickle cell disease is a hereditary disorder characterized by erythrocyte deformity due to hemoglobin polymerization. We assessed in vivo the potential curative threshold of fetal hemoglobin in the SAD transgenic mouse model of sickle cell disease using mating with mice expressing the human fetal Agamma-globin gene. With increasing levels of HbF, AgammaSAD mice showed considerable improvement in all hematologic parameters, morphopathologic features and life span/survival. We established the direct therapeutic effect of fetal hemoglobin on sickle cell disease and demonstrated correction by increasing fetal hemoglobin to about 9-16% in this mouse model. This in vivo study emphasizes the potential of the SAD mouse models for quantitative analysis of gene therapy approaches.

Reduced beta-globin gene expression in adult mice containing deletions of locus control region 5' HS-2 or 5' HS-3

To gain insights into the functions of individual DNA'se hypersensitive sites within the beta globin locus control region (LCR), we deleted the endogenous 5' HS-2 and HS-3 regions from the mouse germline using homologous recombination techniques. We demonstrated that the deletion of either murine 5' HS-2 or 5' HS-3 reduced the expression of the embryonic epsilon y and beta h1 globin genes minimally in yolk sac-derived erythrocytes, but that both knockouts reduced the output of the adult beta (beta-Major + beta-Minor) globin genes by approximately 30% in adult erythrocytes. When the selectable marker PGK-Neo cassette was retained within either the HS-2 or HS-3 region, a much more severe reduction in globin gene expression was observed at all developmental stages. PGK-Neo was shown to be expressed in an erythroid-specific fashion when it was retained in the HS-3 position. These results show that neither 5' HS-2 nor HS-3 is required for the activity of embryonic globin genes, nor are these sites required for correct developmental switching. However, each site is required for approximately 30% of the total LCR activity associated with adult beta-globin gene expression in adult red blood cells. Each site therefore contains some non-redundant information that contributes to adult globin gene function.

Molecular mechanisms of hemoglobin switching

The developmental regulation of the human beta-globin cluster embodies all aspects of transcriptional control of eukaryotic genes. The cis-acting sequences within the cluster, distal regulatory regions and trans-acting factors all contribute to provide stringent temporal and tissue-specific expression. This review will examine the individual regulatory mechanisms which govern globin gene expression and highlight recent advances which expand our understanding of these dynamic interactions.

The position of integration affects expression of the A gamma-globin-encoding gene linked to HS3 in transgenic mice

Proper expression of the human beta-globin (beta Glb) locus is dependent on the presence of a major regulatory element located upstream from the beta Glb gene cluster, the locus control region (LCR). The LCR, as well as the individual DNase-I-hypersensitive sites from which it is composed, have been shown to provide position-of-integration-independent expression in transgenic mice. Here, we report that a transgenic founder carrying multiple integrations of a hypersensitive site 3::A gamma globin gene (HS3::A gamma) construct produced three types of progeny, one with zero A gamma expression in the adult stage, one with minimal A gamma expression (1% of A gamma-expressing cells) and one with abundant A gamma expression (100% A gamma-expressing cells). The possibility that these phenotypes were due to parental imprinting or to DNA rearrangements of the transgene or to point mutations of the HS3 core or the A gamma promoter were excluded. The pattern of inheritance of the three HS3::A gamma transgene phenotypes indicate that the transgene has integrated into three different chromosomes. These results provide direct evidence that the HS3 of the LCR is not sufficient to protect the A gamma gene from position effects excerted by the surrounding chromatin.

Developmental regulation of human beta-globin gene transcription: a switch of loyalties?

Synthesis of different hemoglobin polypeptides during the early stages of human development is principally regulated by transcriptional control mechanisms that determine which of the five beta-type globin genes is expressed. The means by which this is achieved have been scrutinized for several decades, and insights have been gained from introducing segments of the human beta-globin locus into transgenic mice, and from analysis of naturally occurring mutations at the locus. I describe here a model which attempts to resolve several of the current puzzles and provides simple, testable predictions for how differential beta-globin gene transcription might be achieved during human development.

C-myc as an inducer of polycystic kidney disease in transgenic mice

In this study, a genetic model of polycystic kidney disease (PKD) has been produced in transgenic mice bearing the murine c-myc gene driven by the SV40 enhancer and the adult beta-globin promoter. These animals reproducibly develop PKD and die of renal failure. The phenotype appears to result from the overexpression of c-myc in the renal tubular epithelium and consequent abnormal cell proliferation. These transgenic mice represent a genetic model of PKD which bears similarities to human autosomal dominant PKD (ADPKD) with respect to renal morphology, renal functional alterations and dominant transmission. Study of these transgenic mice may offer valuable insights into the pathogenesis of PKD.

Increased protein binding to a -530 mutation of the human beta-globin gene associated with decreased beta-globin synthesis

Although some cases of the syndrome of hereditary persistence of fetal hemoglobin (HPFH) have been correlated with mutations causing a change in the binding of trans-acting factors to DNA sequences flanking the gamma-globin gene, this mechanism has not been described in beta-thalassemias upstream of the canonical promoter of the beta-globin gene. In this report we describe such a change in binding of a protein that may explain a silent carrier phenotype of beta-thalassemia. We have previously demonstrated the binding of a protein (BP1) derived from a nuclear extract of human K562 cells to DNA 5' to the human beta-globin gene in a region having a negative regulatory function. The binding of BP1 in this region can be detected by DNAse I footprinting and by gel mobility shift analysis. We have now compared binding of BP1 to the normal sequence and a mutated sequence (+ATA/-T at -530 bp from the cap site) from the silent carrier of beta-thalassemia. Using mobility shift assays we show that BP1 binds about nine times more strongly to the mutated sequence than the normal sequence. These results suggest the possibility that the decreased expression of the beta-globin gene exhibited by the carrier may be due, at least in part, to tighter binding of a protein which functions as a negative control element or repressor.

Expression of human globin genes in transgenic mice carrying the beta-globin gene cluster with a mutation causing G gamma beta + hereditary persistence of fetal hemoglobin

We have introduced into the mouse germ line the 40-kilobase (kb) Kpn I fragment containing the beta-globin gene cluster from an individual with a non-deletion form of hereditary persistence of fetal hemoglobin (HPFH) believed to be due to a point mutation at position -202 of the G gamma-globin gene. The G gamma-globin gene, as well as the beta-globin gene, was expressed in adult erythroid tissues of the resulting transgenic mice. The level of expression of the G gamma-globin gene was about 20% of that of the beta-globin gene. Others have previously shown that cloned individual normal human beta- and gamma-globin genes containing a limited amount of 5'- and 3'-flanking DNA are expressed in a manner similar to that of their corresponding murine homologs during development in transgenic mice. In contrast, we have observed that the pattern of expression of the normal (non-mutated) A gamma- and beta-globin genes in the 40-kb insert was different from that of their corresponding murine homologs. The beta-globin gene remained inactive at the fetal stage, whereas the normal A gamma-globin gene was expressed beyond the embryonic (yolk sac) stage into the fetal stage of development and then became inactive in adult erythroid cells. The pattern of expression of the human globin transgenes during mouse development resembles that observed during human development. These results suggest that the gross organization of the human beta-like globin gene cluster is important for stage-specific expression of each human globin gene during development.

Regulation of human fetal hemoglobin gene expression

An understanding of the mechanism involved in the regulated expression of the human gamma and beta globin genes requires the detailed definition of the cis-acting DNA sequences and trans-acting protein factors responsible for their developmental stage specific expression. To determine the critical cis-acting elements, hybrid genes containing elements of the gamma and beta globin genes were transfected into K562 cells, a human erythroleukemia line. The regulated expression of the gamma and beta genes was also studied by transferring hybrid genes containing the gamma or beta promoters linked to the neomycin resistance gene (neoR) into erythroid (K562) cells and nonerythroid (Hela) cells. DNA sequences found to be important to the expression of the gamma gene were assayed for the presence of transacting factors by studying the binding of protein factors using the gel mobility shift assay. The results suggest that there are multiple cis-acting elements 5' and 3' to the gamma and beta genes, and perhaps within these genes contributing to their regulation. In addition, there are multiple trans-acting protein factors interacting with these regions which may determine their transcriptional regulation in erythroid cells.

The beta-globin dominant control region activates homologous and heterologous promoters in a tissue-specific manner

We have introduced a human beta-globin minilocus, containing the recently described dominant control region (DCR), the beta-globin or Thy-1 gene, and a thymidine kinase (tk)-neoR gene into erythroid and non-erythroid cells. Analysis of the transcription levels of the genes shows that the DCR directs high levels of human beta-globin, Thy-1 and tk-neo expression independent of integration sites in an erythroid-specific manner. The presence of the DNAasel hypersensitive sites at the 5' end of the locus is required for this effect on the homologous and heterologous gene. An analysis of the DCR chromatin in transfected mouse erythroleukemic cells suggests that the formation of the hypersensitive sites in this region precedes beta-globin gene expression.

A dominant control region from the human beta-globin locus conferring integration site-independent gene expression

The regulatory elements that determine the expression pattern of a number of eukaryotic genes expressed specifically in certain tissues have been defined and studied in detail. In general, however, the expression conferred by these elements on genes reintroduced into the genomes of cell lines and transgenic animals has turned out to be at a low level relative to that of endogenous genes, and influenced by the chromosomal site of insertion of the exogenous construct. We have previously shown that if regions flanking the human beta-globin locus are introduced into the mouse genome along with the human beta-globin gene, a level of expression comparable to that of endogenous genes can be achieved that is also independent of integration site. We have now defined a dominant control region with these properties consisting of 6.5 kilobases of DNA encompassing erythroid cell-specific DNase I hypersensitive sites. The identification of such dominant control regions could have important applications in somatic gene therapy.

Developmental regulation of beta-globin gene switching

A chicken erythroid cell-specific enhancer is located in the intergenic region between the adult beta- and embryonic epsilon-globin genes. In this paper we show that the beta-globin enhancer stimulates transcription of both genes. epsilon-Globin is, however, inappropriately regulated since it is expressed in both embryonic and adult red blood cells. Appropriate stage-specific regulation is observed for both genes when they are present on one plasmid. By analysis of deletion and substitution mutants, we conclude that beta-globin tissue- and developmental stage-specific regulation is mediated by interaction of the beta-globin enhancer with a positive regulatory element within the adult beta-globin promoter, the developmental stage selector element (SSE).