Disease relevance of Hba-x

• In this paper, we show that a number of conserved sequence elements in the zeta-globin promoter are important for promoter activity in transiently transfected K562 erythroleukemia cells, which constitutively express zeta-globin [1].
• Adult individuals who are carriers of the (--SEA/) alpha-thalassemia deletion, in contrast to normal adults, have low levels of embryonic zeta-globin chains in their circulating erythrocytes [2].
• A transgenic mouse strain with the zeta-globin promoter and the vHa-ras oncogene develops an array of mesenchymal and epithelial neoplasms described here [3].
• The Tg.AC mouse carries an activated v-Ha-ras oncogene fused to an embryonic zeta-globin promoter and develops cutaneous papillomas in response to specific chemicals, full thickness wounding, and ultraviolet radiation [4].
• Tg.AC transgenic mice have a transgene composed of a zeta-globin transcriptional control region, a v-Ha-ras coding region, and a simian virus 40 3' polyadenylation signal sequence [5].

High impact information on Hba-x

• Here we show, in vitro and in vivo in a transgenic mouse model of SCD, that the adverse properties of hemoglobin-S can be reversed by exchanging its normal alpha-globin subunits for zeta-globin, an endogenous, developmentally silenced, non-beta-like globin [6].
• In contrast, this binding activity is lacking in the nuclear extract of an embryonic-stage erythroid line expressing zeta-globin [7].
• 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 [7].
• Furthermore, zeta-globin promoter activities in adult mice increase in proportion to the number of integrated DNA fragments even at 19 copies/genome [8].
• We have created a transgenic mouse strain that carries the activated v-Ha-ras oncogene fused to the promoter of the mouse embryonic alpha-like, zeta-globin gene [9].

Biological context of Hba-x

• This element is localized 26 kb upstream of the mouse embryonic globin gene (Hba-x) and is highly conserved at the protein binding sites [10].
• The alpha-HVR is located approximately 18 kb upstream of the mouse zeta-globin gene transcription start site and contains a variable copy number tandem repeat (VNTR) array of a 35-bp sequence rich in (G+C) content [11].
• Interestingly, the zeta-globin gene is also active when permanently transfected into an immunoglobulin-producing B-cell, a cell that presumably has tissue-specific requirements for gene expression [12].
• The structure of this gene establishes the amino acid sequence of the mouse embryonic zeta-globin polypeptide chain and allows us to identify sequences within the gene that may be important for its expression [12].
• Consistent with previous studies, expression of the individual alpha- and zeta-globin transgenes was found to be developmentally appropriate [13].

Anatomical context of Hba-x

• By introducing the cloned gene into permanently transfected mouse erythroleukemic cell lines and comparing its transcript with that of zeta-globin mRNA derived from embryonic yolk sac erythrocytes, we are able to show that the cloned gene is transcriptionally active and that its transcript is correctly initiated and processed [12].
• To test this possibility, we created a null mutation of the zeta-globin gene using homologous recombination in embryonic stem cells [14].
• The zeta-globin transcribed region appeared to contribute to gene silencing by a mechanism specifically activated in definitive erythroblasts in the fetal liver [13].
• Subsequently, zeta-globin chains are present in fetal blood at a very low level, which is supplanted by the expression of alpha-globin chains [2].
• In the human EBs, we could show a characteristic regional expression of embryonic markers specific to different cellular lineages, namely, zeta-globin (mesoderm), neurofilament 68Kd (ectoderm), and alpha-fetoprotein (endoderm) [15].

Associations of Hba-x with chemical compounds

• Effect of zeta-globin substitution on the O2-transport properties of Hb S in vitro and in vivo [16].
• In contrast, retinoic acid blocks the expression of the mesodermal genes Brachyury, cardiac actin, and zeta-globin [17].

Regulatory relationships of Hba-x

• Interestingly, insertion of the PGK-Neo cassette (used to create the null mutation) into the zeta-globin gene appears to influence the expression of the nearby alpha-globin genes, giving rise to reduced alpha-globin production and to an alpha-thalassemia-like syndrome [14].
• It is shown that tumor necrosis factor-alpha represses the transcription activity directed by either alpha or zeta globin promoter in a dose-dependent manner [18].

Other interactions of Hba-x

• This finding raises the possibility that zeta-globin production might be wholly or partially redundant in embryos in which the adult alpha-globin is also expressed [14].
• 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 [19].
• 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 [13].
• 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 [19].
• To identify drugs that might provoke a false positive response in the Tg.AC assay, we screened pharmaceuticals for in vitro induction of the gadd153 promoter and the zeta-globin promoter [20].

Analytical, diagnostic and therapeutic context of Hba-x

• Molecular cloning and sequence analysis of a cDNA coding for the mouse alpha-like embryonic globin chain x [21].
• Southern blot hybridization of genomic DNA from 66 tail biopsies using a zeta-globin probe revealed that all of the hemizygous [22].

References