Long-range regulation of ?? globin gene expression during erythropoiesis

Adventitious changes in long-range gene expression caused by polymorphic structural variation and promoter competition

It is well established that all of the cis-acting sequences required for fully regulated human alpha-globin expression are contained within a region of approximately 120 kb of conserved synteny. Here, we show that activation of this cluster in erythroid cells dramatically affects expression of apparently unrelated and noncontiguous genes in the 500 kb surrounding this domain, including a gene (NME4) located 300 kb from the alpha-globin cluster. Changes in NME4 expression are mediated by physical cis-interactions between this gene and the alpha-globin regulatory elements. Polymorphic structural variation within the globin cluster, altering the number of alpha-globin genes, affects the pattern of NME4 expression by altering the competition for the shared alpha-globin regulatory elements. These findings challenge the concept that the genome is organized into discrete, insulated regulatory domains. In addition, this work has important implications for our understanding of genome evolution, the interpretation of genome-wide expression, expression-quantitative trait loci, and copy number variant analyses.

Erythropoiesis: model systems, molecular regulators, and developmental programs

Human erythropoiesis is a complex multistep developmental process that begins at the level of pluripotent hematopoietic stem cells (HSCs) at bone marrow microenvironment (HSCs niche) and terminates with the production of erythrocytes (RBCs). This review covers the basic and contemporary aspects of erythropoiesis. These include the: (a) cell-lineage restricted pathways of differentiation originated from HSCs and going downward toward the blood cell development; (b) model systems employed to study erythropoiesis in culture (erythroleukemia cell lines and embryonic stem cells) and in vivo (knockout animals: avian, mice, zebrafish, and xenopus); (c) key regulators of erythropoiesis (iron, hypoxia, stress, and growth factors); (d) signaling pathways operating at hematopoietic stem cell niche for homeostatic regulation of self renewal (SCF/c-kit receptor, Wnt, Notch, and Hox) and for erythroid differentiation (HIF and EpoR). Furthermore, this review presents the mechanisms through which transcriptional factors (GATA-1, FOG-1, TAL-1/SCL/MO2/Ldb1/E2A, EKLF, Gfi-1b, and BCL11A) and miRNAs regulate gene pattern expression during erythroid differentiation. New insights regarding the transcriptional regulation of alpha- and beta-globin gene clusters were also presented. Emphasis was also given on (i) the developmental program of erythropoiesis, which consists of commitment to terminal erythroid maturation and hemoglobin production, (two closely coordinated events of erythropoieis) and (ii) the capacity of human embryonic and umbilical cord blood (UCB) stem cells to differentiate and produce RBCs in culture with highly selective media. These most recent developments will eventually permit customized red blood cell production needed for transfusion.

Chromosome looping at the human alpha-globin locus is mediated via the major upstream regulatory element (HS -40)

Previous studies in the mouse have shown that high levels of alpha-globin gene expression in late erythropoiesis depend on long-range, physical interactions between remote upstream regulatory elements and the globin promoters. Using quantitative chromosome conformation capture (q3C), we have now analyzed all interactions between 4 such elements lying 10 to 50 kb upstream of the human alpha cluster and their interactions with the alpha-globin promoter. All of these elements interact with the alpha-globin gene in an erythroid-specific manner. These results were confirmed in a mouse model of human alpha globin expression in which the human cluster replaces the mouse cluster in situ (humanized mouse). We have also shown that expression and all of the long-range interactions depend largely on just one of these elements; removal of the previously characterized major regulatory element (called HS -40) results in loss of all the interactions and alpha-globin expression. Reinsertion of this element at an ectopic location restores both expression and the intralocus interactions. In contrast to other more complex systems involving multiple upstream elements and promoters, analysis of the human alpha-globin cluster during erythropoiesis provides a simple and tractable model to understand the mechanisms underlying long-range gene regulation.

Chaperoning erythropoiesis

Multisubunit complexes containing molecular chaperones regulate protein production, stability, and degradation in virtually every cell type. We are beginning to recognize how generalized and tissue-specific chaperones regulate specialized aspects of erythropoiesis. For example, chaperones intersect with erythropoietin signaling pathways to protect erythroid precursors against apoptosis. Molecular chaperones also participate in hemoglobin synthesis, both directly and indirectly. Current knowledge in these areas only scratches the surface of what is to be learned. Improved understanding of how molecular chaperones regulate erythropoietic development and hemoglobin homeostasis should identify biochemical pathways amenable to pharmacologic manipulation in a variety of red blood cell disorders including thalassemia and other anemias associated with hemoglobin instability.

A large deletion in the human alpha-globin cluster caused by a replication error is associated with an unexpectedly mild phenotype

We have characterized a newly identified 16.6 kb deletion which removes a significant proportion of the human alpha-globin cluster including the psizeta1, alpha(D), psialpha1 and alpha2-globin genes but leaves the duplicated alpha1 gene intact. This complicated rearrangement results from a combination of slippage and strand switching at sites of microhomology during replication. Functional analysis shows that expression of the remaining alpha1 gene is increased, rather than down-regulated by this deletion. This could be related to its proximity to the remote upstream alpha-globin regulatory elements or reduced competition for these elements in the absence of the dominant alpha2-globin gene. The finding of a very mild phenotype associated with such an extensive deletion in the alpha-globin cluster implies that much of the DNA removed by the deletion is likely to be functionally unimportant. These findings suggest that other than the upstream regulatory elements and promoter proximal elements there are unlikely to be additional positive cis-acting sequences in the alpha-globin cluster.