Human alpha-globin genes demonstrate autonomous developmental regulation in transgenic mice

Fetal gene therapy of alpha-thalassemia in a mouse model

Fetuses with homozygous alpha-thalassemia usually die at the third trimester of pregnancy or soon after birth. Hence, the disease could potentially be a target for fetal gene therapy. We have previously established a mouse model of alpha-thalassemia. These mice mimic the human alpha-thalassemic conditions and can be used as preclinical models for fetal gene therapy. We tested a lentiviral vector containing the HS 2, 3, and 4 of the beta-LCR, a central polypurine tract element, and the beta-globin gene promoter directing either the EGFP or the human alpha-globin gene. We showed that the GFP expression was erythroid-specific and detected in BFU-E colonies and the erythroid progenies of CFU-GEMM. For in utero gene delivery, we did yolk sac vessel injection at midgestation of mouse embryos. The recipient mice were analyzed after birth for human alpha-globin gene expression. In the newborn, human alpha-globin gene expression was detected in the liver, spleen, and peripheral blood. The human alpha-globin gene expression was at the peak at 3-4 months, when it reached 20% in some recipients. However, the expression declined at 7 months. Colony-forming assays in these mice showed low abundance of the transduced human alpha-globin gene in their BFU-E and CFU-GEMM and the lack of its transcript. Thus, lentiviral vectors can be an effective vehicle for delivering the human alpha-globin gene into erythroid cells in utero, but, in the mouse model, delivery at late midgestation could not transduce hematopoietic stem cells adequately to sustain gene expression.

The control of expression of the alpha-globin gene cluster

The alpha-globin gene cluster is located at the very tip of the short arm of chromosome 16. It produces the alpha-like globins, which is combined with the beta-like globins to form hemoglobin, and its mutants cause alpha-thalassemia, which is one of the most common genetic diseases. Its expression shows a tissue and developmental stage specificity that is balanced with that of the beta-globin gene cluster. In this article, we summarize the research on the control of expression of the alpha-globin gene cluster, mainly with respect to the alpha-major regulatory element (alpha-MRE): HS-40, the tissue-specific and developmental control of its expression, and its chromosomal environment. In summary, the alpha-globin gene cluster is expressed in an open chromosomal environment; HS-40, the 5'-flanking sequence, the transcribed region, and the 3'-flanking sequence interact to fully regulate its expression.

A 3'-flanking NF-kappaB site mediates developmental silencing of the human zeta-globin gene

The central developmental event in the human (h)alpha-globin gene cluster is selective silencing of the zeta-globin gene as erythropoiesis shifts from primitive erythroblasts in the embryonic yolk sac to definitive erythroblasts in the fetal liver. Previous studies have demonstrated that full developmental silencing of the hzeta-globin gene in transgenic mice requires the proximal 2.1 kb of its 3'-flanking region. In the current report, we localize this silencing activity to a 108 bp segment located 1.2 kb 3' to the zeta-globin gene. Protein(s) in nuclear extracts from cell lines representing the fetal/adult erythroid stage bind specifically to an NF-kappaB motif located at this site. In contrast, this binding activity is lacking in the nuclear extract of an embryonic-stage erythroid line expressing zeta-globin. This complex is quantitatively recognized by antisera to the NF-kappaB p50 and to a lesser extent to p65 subunits. A two-base substitution that disrupts NF-kappaB site protein binding in vitro also results in the loss of the developmental silencing activity in vivo. The data suggest that NF-kappaB complex formation is a crucial component of hzeta-globin gene silencing. This finding expands the roles of this widely distributed transcriptional complex to include negative regulation in mammalian development.

Derepression of human embryonic zeta-globin promoter by a locus-control region sequence

A multiple protein-DNA complex formed at a human alpha-globin locus-specific regulatory element, HS-40, confers appropriate developmental expression pattern on human embryonic zeta-globin promoter activity in humans and transgenic mice. We show here that introduction of a 1-bp mutation in an NF-E2/AP1 sequence motif converts HS-40 into an erythroid-specific locus-control region. Cis-linkage with this locus-control region, in contrast to the wild-type HS-40, allows erythroid lineage-specific derepression of the silenced human zeta-globin promoter in fetal and adult transgenic mice. Furthermore, zeta-globin promoter activities in adult mice increase in proportion to the number of integrated DNA fragments even at 19 copies/genome. The mutant HS-40 in conjunction with human zeta-globin promoter thus can be used to direct position-independent and copy number-dependent expression of transgenes in adult erythroid cells. The data also supports a model in which competitive DNA binding of different members of the NF-E2/AP1 transcription factor family modulates the developmental stage specificity of an erythroid enhancer. Feasibility to reswitch on embryonic/fetal globin genes through the manipulation of nuclear factor binding at a single regulatory DNA motif is discussed.

Proximal promoter elements of the human zeta-globin gene confer embryonic-specific expression on a linked reporter gene in transgenic mice

We have investigated the transcriptional regulation of the human embryonic zeta-globin gene promoter. First, we examined the effect that deletion of sequences 5' to zeta-globin's CCAAT box have on zeta-promoter activity in erythroid cell lines. Deletions of sequences between -116 and -556 (cap = 0) had little effect while further deletion to -84 reduced zeta-promoter activity by only 2-3-fold in both transiently and stably transfected erythroid cells. Constructs containing 67, 84 and 556 bp of zeta-globin 5' flanking region linked to a beta-galactosidase reporter gene (lacZ) and hypersensitive site -40 (HS-40) of the human alpha-globin gene cluster were then employed for the generation of transgenic mice. LacZ expression from all constructs, including a 67 bp zeta-globin promoter, was erythroid-specific and most active between 8.5 and 10.5 days post-fertilisation. By 16.5 days gestation, lacZ expression dropped 40-100-fold. These results suggest that embryonic-specific activation of the human zeta-globin promoter is conferred by a 67 bp zeta-promoter fragment containing only a CCAAT and TATA box.

Developmental silencing of the embryonic zeta-globin gene: concerted action of the promoter and the 3'-flanking region combined with stage-specific silencing by the transcribed segment

Globin gene switching is a well-described model of eucaryotic developmental control. In the case of the human alpha-globin gene cluster, migration of erythropoietic activity from the embryonic yolk sac to the fetal liver is parallaled by the zeta-globin gene silencing and enhanced expression of the alpha-globin genes. To map critical cis determinants of this switch, the human zeta-globin gene, the alpha-globin gene, and chimeric recombinants were introduced into the mouse genome. Consistent with previous studies, expression of the individual alpha- and zeta-globin transgenes was found to be developmentally appropriate. Contrary to current models, however, the alpha- and zeta-globin gene promoters were not sufficient to establish this control. Instead, full silencing of the zeta-globin gene required the combined activities of this promoter, transcribed region, and 3'-flanking sequences. Individually, the silencing activities of the zeta-globin gene promoter and 3'-flanking region were minimal but increased markedly when both regions were present. The zeta-globin transcribed region appeared to contribute to gene silencing by a mechanism specifically activated in definitive erythroblasts in the fetal liver. These data demonstrate that a complex set of controls, requiring at least three determinants and involving at least two independent mechanisms, is necessary for full developmental silencing of the human zeta-globin gene.

Invasion and maintenance of a gene duplication

The ubiquity of multigene families is evidence for the frequent occurrence of gene duplication, but the origin of multigene families from a single gene remains a little-studied aspect of genome evolution. Although it is clear that a duplication can arise and become fixed in a population purely by random genetic drift and that the rate of fixation is accelerated if the duplication is directly advantageous, the nature of gene duplication suggests that other factors may influence the fate of a novel duplication. In the face of disadvantageous loss-of-function mutations, duplication of a functional gene may provide a buffer against such mutations. Here the conditions for invasion of a rare duplication starting from a mutation-selection balance are derived with formal population genetic models in both haploids and diploids. Recurrent duplication protects the duplicated chromosome from loss and can be very effective in increasing its frequency in a population. In the absence of recurrent duplication, one might suppose that a duplication would be favored by natural selection because it can mask the effects of deleterious mutations. However, the models show that a duplication can invade only if it provides a direct advantage to the organism. This result is closely related to recent theoretical work on the evolution of ploidy.

Analysis of the human alpha-globin gene cluster in transgenic mice

A 350-bp segment of DNA associated with an erythroid-specific DNase I-hypersensitive site (HS-40), upstream of the alpha-globin gene cluster, has been identified as the major tissue-specific regulator of the alpha-globin genes. However, this element does not direct copy number-dependent or developmentally stable expression of the human genes in transgenic mice. To determine whether additional upstream hypersensitive sites could provide more complete regulation of alpha gene expression we have studied 17 lines of transgenic mice bearing various DNA fragments containing HSs -33, -10, -8, and -4, in addition to HS -40. Position-independent, high-level expression of the human zeta- and alpha-globin genes was consistently observed in embryonic erythroid cells. However, the additional HSs did not confer copy-number dependence, alter the level of expression, or prevent the variable down-regulation of expression in adults. These results suggest that the region upstream of the human alpha-globin genes is not equivalent to that upstream of the beta locus and that although the two clusters are coordinately expressed, there may be differences in their regulation.

The developmental regulation of the human zeta-globin gene in transgenic mice employing beta-galactosidase as a reporter gene

We have investigated the developmental and tissue specific expression of the human embryonic zeta-globin gene in transgenic mice. A construct containing 550 bp of zeta-globin 5' flanking region, fused to a beta-galactosidase (lacZ) reporter gene and linked to the locus control region (LCR)-like alpha positive regulatory element (alpha PRE) was employed for the production of transgenic mice. Firstly, we compared the number of live born transgenic mice containing this construct to the number of live born transgenic mice containing the entire zeta-globin gene linked to the alpha PRE or the beta LCR. Data showed that 12% of mice generated from eggs injected with zeta-promoter/lacZ/alpha PRE DNA were transgenic compared to only 2% of mice generated from eggs injected with the entire zeta-globin gene linked to the alpha PRE or the beta LCR. The reduced number of live born transgenic mice containing the latter constructs suggests that death of transgenic embryos, possibly due to thalassaemia, may be occurring. X-gal staining of whole embryos containing the lacZ gene revealed that zeta-globin promoter activity was most pronounced at 8.5-9.5 days of development and was restricted to erythroid cells. By 15 days of development, no zeta-globin promoter activity was detected. These results suggest that the alpha PRE can direct high level expression from the zeta-globin promoter and that sequences required for the correct tissue and developmental specific expression of the human zeta-globin gene are present within 550 bp's of 5' flanking region. Sequences within the body of the zeta-globin gene or 3' of the cap site do not appear to be necessary for correct zeta-globin developmental regulation.