Promoters of the murine embryonic beta-like globin genes Ey and betah1 do not compete for interaction with the beta-globin locus control region

Mammalian beta-globin loci contain multiple beta-like genes that are expressed at different times during development. The murine beta-globin locus contains two genes expressed during the embryo stage, Ey and betah1, and two genes expressed at both the fetal and postnatal stages, beta-major and beta-minor. Studies of transgenic human beta-like globin loci in mice have suggested that expression of one gene at the locus will suppress expression of other genes at the locus. To test this hypothesis we produced mouse lines with deletions of either the Ey or betah1 promoter in the endogenous murine beta-globin locus. Promoter deletion eliminated expression of the mutant gene but did not affect expression of the remaining embryonic gene or the fetal-adult beta-globin genes on the mutant allele. These results demonstrate a lack of competitive effects between individual mouse embryonic beta-globin gene promoters and other genes in the locus. The implication of these findings for models of beta-globin gene expression are discussed.     

Genomic evidence for independent origins of beta-like globin genes in monotremes and therian mammals

Phylogenetic reconstructions of the beta-globin gene family in vertebrates have revealed that developmentally regulated systems of hemoglobin synthesis have been reinvented multiple times in independent lineages. For example, the functional differentiation of embryonic and adult beta-like globin genes occurred independently in birds and mammals. In both taxa, the embryonic beta-globin gene is exclusively expressed in primitive erythroid cells derived from the yolk sac. However, the "epsilon-globin" gene in birds is not orthologous to the epsilon-globin gene in mammals, because they are independently derived from lineage-specific duplications of a proto beta-globin gene. Here, we report evidence that the early and late expressed beta-like globin genes in monotremes and therian mammals (marsupials and placental mammals) are the products of independent duplications of a proto beta-globin gene in each of these two lineages. Results of our analysis of genomic sequence data from a large number of vertebrate taxa, including sequence from the recently completed platypus genome, reveal that the epsilon- and beta-globin genes of therian mammals arose via duplication of a proto beta-globin gene after the therian/monotreme split. Our analysis of genomic sequence from the platypus also revealed the presence of a duplicate pair of beta-like globin genes that originated via duplication of a proto beta-globin gene in the monotreme lineage. This discovery provides evidence that, in different lineages of mammals, descendent copies of the same proto beta-globin gene may have been independently neofunctionalized to perform physiological tasks associated with oxygen uptake and storage during embryonic development.     

Erythropoietin critically regulates the terminal maturation of murine and human primitive erythroblasts

Primitive erythroid cells, the first red blood cells produced in the mammalian embryo, are necessary for embryonic survival. Erythropoietin and its receptor EpoR, are absolutely required for survival of late-stage definitive erythroid progenitors in the fetal liver and adult bone marrow. Epo- and Epor-null mice die at E13.5 with a lack of definitive erythrocytes. However, the persistence of circulating primitive erythroblasts raises questions about the role of erythropoietin/EpoR in primitive erythropoiesis. Using Epor-null mice and a novel primitive erythroid 2-step culture we found that erythropoietin is not necessary for specification of primitive erythroid progenitors. However, Epor-null embryos develop a progressive, profound anemia by E12.5 as primitive erythroblasts mature as a synchronous cohort. This anemia results from reduced primitive erythroblast proliferation associated with increased p27 expression, from advanced cellular maturation, and from markedly elevated rates of apoptosis associated with an imbalance in pro- and anti-apoptotic gene expression. Both mouse and human primitive erythroblasts cultured without erythropoietin also undergo accelerated maturation and apoptosis at later stages of maturation. We conclude that erythropoietin plays an evolutionarily conserved role in promoting the proliferation, survival, and appropriate timing of terminal maturation of primitive erythroid precursors.     

Cytosine arabinoside plus hemin treatment of a human erythroid cell line, KMOE, strongly induces embryonic, fetal, and adult beta-like globin genes

A variety of regimens were utilized on KMOE cells to maximally raise globin mRNA levels for the purpose of improving the usefullness of this line for globin gene studies. Steady-state mRNA levels of embryonic (epsilon), fetal (gamma) and adult (beta) globin genes were assayed by the S1-nuclease protection method before and after exposure to inducing compounds. Exposure of KMOE cells to cytosine arabinoside and hemin leads to over 20-fold increases in beta- and gamma-globin mRNA steady-state levels, and an over 60-fold increase in epsilon-globin mRNA level. Exposure to cytosine arabinoside alone induced beta- and epsilon-globin but not gamma-globin gene expression. The alpha-like globin genes (zeta and alpha) were also monitored but found to be poorly expressed and not significantly inducible. The presence of epsilon-globin mRNA and the lack of alpha-globin mRNA distinguishes this line, KMOE-EL, from the KMOE sublines previously described.     

Regulation of human embryonic globin genes zeta 2 and epsilon in stably transformed mouse erythroleukemia cells.

Previous work has suggested that the promoter regions of the human embryonic zeta 2 and epsilon globin genes contain negative regulatory regions that could play a role in the repression of these genes in postembryonic erythroblasts. We have examined this possibility by studying the expression of these genes in mouse erythroleukemia cells, an adult erythroid cell line that might be expected to contain repressor molecules that would bind to the putative negative regulatory regions. When attached to appropriate upstream regulatory elements (alpha HS-40 and beta HS1,2) both the zeta and epsilon genes were expressed in these cells at a low level, but no increase in expression was observed when similar constructs lacking the proposed negative regulatory sequences were introduced into these cells. These results cast doubt on the possibility that these sequences play a major role in the developmental repression of the embryonic globin genes, unless they function only in a normal chromosomal organization.     

Short-chain fatty acid derivatives induce fetal globin expression and erythropoiesis in vivo

Orally bioactive compounds that induce gamma globin gene expression at tolerable doses are needed for optimal treatment of the beta-hemoglobinopathies. Short-chain fatty acids (SCFAs) of 2 to 6 carbons in length induce gamma globin expression in animal models, and butyrate, phenylbutyrate, and valproate induce gamma globin in human patients. The usefulness of these compounds, however, is limited by requirements for large doses because of their rapid metabolism and their tendency to inhibit cell proliferation, which limits the pool of erythroid progenitors in which gamma globin can be induced. Selected short-chain fatty acid derivatives (SCFADs) were recently found to induce gamma globin and to stimulate the proliferation of hematopoietic cells in vitro. These SCFADs are now evaluated in vivo in nonanemic transgenic mice containing the human beta globin gene locus and in anemic phlebotomized baboons. In mice treated with a SCFAD once daily for 5 days, gamma globin mRNA increased 2-fold, reticulocytes increased 3- to 7-fold, and hematocrit levels increased by 27%. Administration of 3 SCFADs in anemic baboons increased F-reticulocytes 2- to 15-fold over baseline and increased total hemoglobin levels by 1 to 2 g/dL per week despite ongoing significant daily phlebotomy. Pharmacokinetic studies demonstrated 90% oral bioavailability of 2 SCFADs, and targeted plasma levels were maintained for several hours after single oral doses equivalent to 10% to 20% of doses required for butyrate. These findings identify SCFADs that stimulate gamma globin gene expression and erythropoiesis in vivo, activities that are synergistically beneficial for treatment of the beta hemoglobinopathies and useful for the oral treatment of other anemias.     

Erythropoietin receptor signaling regulates both erythropoiesis and megakaryopoiesis in vivo

Transgenic expression of a gain-of-function truncated mouse erythropoietin receptor gene (EpoR) leads to expansion of the HSC pool in response to human erythropoietin (Epo). We have re-examined this observation using a knock-in mouse model, wherein the mouse EpoR gene was replaced in its proper genetic locus by a single copy of either a wild-type human or a polycythemia-inducing truncated human EPOR gene. Bone marrow cells obtained from knock-in mice were transplanted together with competitor bone marrow cells in a model that allows tracking of erythroid, platelet, and leukocyte contributions by each genotype. Secondary transplants were also performed. Stem/progenitor cells were identified phenotypically and isolated for colony-forming assays to evaluate cytokine responsiveness by cells with the wild-type human or truncated human EPOR gene. Augmented Epo signaling increased erythroid repopulation post-transplant as expected, but had no effect on short-term or long-term leukocyte repopulation. However, the wild-type human EPOR knock-in mouse showed decreases in both erythroid and platelet repopulation compared to marrow cells from the mutant human EPOR knock-in mouse or normal B6 animals. These results provide evidence supporting a role for Epo signaling in megakaryopoiesis in vivo and suggest a role for Epo signaling early in hematopoietic development.     

Linkage of genes for adult alpha-globin and embryonic alpha-like globin chains.

In alpha-thalassemia, the genetic locus for the alpha chains of adult hemoglobin is not expressed. We have examined the hemoglobins of a number of individual mouse embryos heterozygous for a particular alpha-thalassemia (Hbath-J) and find no decrease in the proportion of hemoglobins containing the alpha chain as compared to the hemoglobin containing the alpha-like embryonic globin chain. This result suggests that the locus for this embryonic alpha-like chain is inactivated or deleted in these embryos as well. Because a single mutational event inactivated adult and embryonic loci, we conclude that they are probably closely linked to one another on the same chromosome. We also present evidence that an unusual hemoglobin in the blood of these embryos is composed only of an embryonic beta-like chain, and is thus analogous to the hemoglobin H (beta 4 tetramer) of adult alpha-thalassemics.