The control of expression of the alpha-globin gene cluster

Two large novel alpha-globin gene cluster deletions causing alpha(0)-thalassemia in two Chinese families

INTRODUCTION

Monosomy of terminal 16p13.3 is a relatively common subtelomeric abnormality, most affected individuals presented α-thalassemia, some also have mental retardation, developmental abnormalities and/or speech delay and facial dysmorphism, which is termed ATR-16 syndrome. Here, we reported two novel 16p13.3 deletions involving the α-globin gene cluster and multispecies conserved sequences (MCSs), causing only a phenotype of α-thalassemia.

METHODS

Samples were collected from members of the two families and were subjected to haematological and comprehensive genetic analysis.

RESULTS

The novel 108 Kb deletion in family A extends from the non-protein coding RNA gene (WASIR2) to the NPRL3 gene, removing MCS-R1 to R3. This deletion should arise de novo because it wasn't detected in both parents. The novel 336 Kb deletion in family B should extend from telomere to ∼ chr16:336000, removing the entire α-globin gene cluster. Carriers of these two deletions presented with microcytosis and hypochromic red cells, in accordance with a phenotype of α⁰-thalassemia carrier.

CONCLUSION

Our study increases the mutation spectrum of α-thalassemia. MCSs deletion should be considered in clinical practice of thalassemia screening and diagnosis.

KRAB-Zinc Finger Proteins: A Repressor Family Displaying Multiple Biological Functions

Zinc finger proteins containing the Kruppel associated box (KRAB-ZFPs) constitute the largest individual family of transcriptional repressors encoded by the genomes of higher organisms. KRAB domain, positioned at the NH2 terminus of the KRAB-ZFPs, interacts with a scaffold protein, KAP-1, which is able to recruit various transcriptional factors causing repression of genes to which KRAB ZFPs bind. The relevance of such repression is reflected in the large number of the KRAB zinc finger protein genes in the human genome. However, in spite of their numerical abundance little is currently known about the gene targets and the physiological functions of KRAB- ZFPs. However, emerging evidence links the transcriptional repression mediated by the KRAB-ZFPs to cell proliferation, differentiation, apoptosis and cancer. Moreover, the fact that KRAB containing proteins are vertebrate-specific suggests that they have evolved recently, and that their key roles lie in some aspects of vertebrate development. In this review, we will briefly discuss some regulatory functions of the KRAB-ZFPs in different physiological and pathological states, thus contributing to better understand their biological roles.

Gene order in human alpha-globin locus is required for their temporal specific expressions

The human alpha-globin cluster represents a unique model of transcriptional regulation and provides challenges to the current understanding of interactions between distal and proximal regulatory elements. Although the gene proximal regions are believed to possess almost all the necessary elements for temporal and spatial specificity of gene transcription, it is still not clear whether the relative distance of embryonic zeta- and fetal/adult alpha-genes to their distal regulatory element alpha-URE plays any role in transcriptional switching. To investigate the role of gene order in regulating temporal expression, we inverted the entire structure gene region of human alpha-globin locus in a BAC clone bringing alpha-genes closest to alpha-URE and zeta-gene the farthest away. Expression analysis of the reverted locus in transgenic mice showed that alpha-globin genes, now relocated closer to alpha-URE, maintained their expression levels through all developmental stages. However, the zeta-globin gene suffered a total loss at both embryonic and fetal/adult stages. It indicates that proximal location of zeta-globin gene to alpha-URE is necessary for its normal embryonic expression and necessary to prevent embryonic expression of the alpha-globin gene. We proved that, in the human alpha-globin gene cluster, the normal order of structural genes relative to alpha-URE plays a crucial role in the regulation of developmental switching.

A comprehensive catalog of human KRAB-associated zinc finger genes: insights into the evolutionary history of a large family of transcriptional repressors

Krüppel-type zinc finger (ZNF) motifs are prevalent components of transcription factor proteins in all eukaryotes. KRAB-ZNF proteins, in which a potent repressor domain is attached to a tandem array of DNA-binding zinc-finger motifs, are specific to tetrapod vertebrates and represent the largest class of ZNF proteins in mammals. To define the full repertoire of human KRAB-ZNF proteins, we searched the genome sequence for key motifs and then constructed and manually curated gene models incorporating those sequences. The resulting gene catalog contains 423 KRAB-ZNF protein-coding loci, yielding alternative transcripts that altogether predict at least 742 structurally distinct proteins. Active rounds of segmental duplication, involving single genes or larger regions and including both tandem and distributed duplication events, have driven the expansion of this mammalian gene family. Comparisons between the human genes and ZNF loci mined from the draft mouse, dog, and chimpanzee genomes not only identified 103 KRAB-ZNF genes that are conserved in mammals but also highlighted a substantial level of lineage-specific change; at least 136 KRAB-ZNF coding genes are primate specific, including many recent duplicates. KRAB-ZNF genes are widely expressed and clustered genes are typically not coregulated, indicating that paralogs have evolved to fill roles in many different biological processes. To facilitate further study, we have developed a Web-based public resource with access to gene models, sequences, and other data, including visualization tools to provide genomic context and interaction with other public data sets.

Heme-dependent up-regulation of the α-globin gene expression by transcriptional repressor Bach1 in erythroid cells

The transcriptional factor Bach1 forms a heterodimer with small Maf family, and functions as a repressor of the Maf recognition element (MARE) in vivo. To investigate the involvement of Bach1 in the heme-dependent regulation of the expression of the alpha-globin gene, human erythroleukemia K562 cells were cultured with succinylacetone (SA), a heme biosynthetic inhibitor, and the level of alpha-globin mRNA was examined. A decrease of alpha-globin mRNA was observed in SA-treated cells, which was restored by the addition of hemin. The heme-dependent expression of alpha-globin occurred at the transcriptional level since the expression of human alpha-globin gene promoter-reporter gene containing hypersensitive site-40 (HS-40) was decreased when K562 cells were cultured with SA. Hemin treatment restored the decrease of the promoter activity by SA. The regulation of the HS-40 activity by heme was dependent on the NF-E2/AP-1 (NA) site, which is similar to MARE. The NA site-binding activity of Bach1 in K562 increased upon SA-treatment, and the increase was diminished by the addition of hemin. The transient expression of Bach1 and mutated Bach1 lacking CP motifs suppressed the HS-40 activity, and cancellation of the repressor activity by hemin was observed when wild-type Bach1 was expressed. The expression of NF-E2 strengthened the restoration of the Bach1-effect by hemin. Interestingly, nuclear localization of Bach1 increased when cells were treated with SA, while hemin induced the nuclear export of Bach1. These results indicated that heme plays an important role in the induction of alpha-globin gene expression through disrupting the interaction of Bach1 and the NA site in HS-40 enhancer in erythroid cells.