Transcriptional networks orchestrate complex developmental processes. Such networks are commonly instigated by master regulators of development. Considerable progress has been made in elucidating GATA factor-dependent genetic networks that control blood cell development. GATA-2 is required for the genesis and/or function of hematopoietic stem cells, whereas GATA-1 drives the differentiation of hematopoietic progenitors into a subset of the blood cell lineages. GATA-1 directly represses Gata2 transcription, and this involves GATA-1-mediated displacement of GATA-2 from chromatin, a process termed a GATA switch. GATA switches occur at numerous loci with critical functions, indicating that they are widely utilized developmental control tools.

Numerous examples exist of how disrupting the actions of physiological regulators of blood cell development yields hematologic malignancies. The master regulator of hematopoietic stem/progenitor cells GATA-2 was cloned almost 20 years ago, and elegant genetic analyses demonstrated its essential function to promote hematopoiesis. While certain GATA-2 target genes are implicated in leukemogenesis, only recently have definitive insights emerged linking GATA-2 to human hematologic pathophysiologies. These pathophysiologies include myelodysplastic syndrome, acute myeloid leukemia and an immunodeficiency syndrome with complex phenotypes including leukemia. As GATA-2 has a pivotal role in the etiology of human cancer, it is instructive to consider mechanisms underlying normal GATA factor function/regulation and how dissecting such mechanisms may reveal unique opportunities for thwarting GATA-2-dependent processes in a therapeutic context. This article highlights GATA factor mechanistic principles, with a heavy emphasis on GATA-1 and GATA-2 functions in the hematopoietic system, and new links between GATA-2 dysregulation and human pathophysiologies.

SummaryMany acute and chronic anemias, including hemolysis, sepsis, and genetic bone marrow failure diseases such as Diamond-Blackfan Anemia (DBA), are not treatable with erythropoietin (Epo), because the colony-forming unit erythroid progenitors (CFU-Es) that respond to Epo are either too few in number or are not sensitive enough to Epo to maintain sufficient red blood cell production 1,2,3–5,6,7,8,9. Treatment of these anemias requires a drug that acts at an earlier stage of red cell formation and enhances the formation of Epo-sensitive CFU-E progenitors. Recently we showed that glucocorticoids specifically stimulate self-renewal of the early erythroid progenitor, the burst-forming unit erythroid (BFU-E), and increase the production of terminally differentiated erythroid cells 10,11. Here we demonstrate that activation of the peroxisome proliferator-activated receptor alpha (PPARα) by PPARα agonists, GW7647 and fenofibrate, synergizes with glucocorticoid receptor (GR) to promote BFU-E self-renewal. Over time these agonists greatly increase production of mature red blood cells in cultures both of mouse fetal liver BFU-Es and of mobilized human adult CD34+ peripheral blood progenitors, the latter employing a new and effective culture system that generates normal enucleated reticulocytes. While PPARα−/− mice show no hematological difference from wild-type mice in both normal and phenylhydrazine (PHZ)-induced stress erythropoiesis, PPARα agonists facilitate recovery of wild-type mice, but not PPARα−/− mice, from PHZ-induced acute hemolytic anemia. We also showed that PPARα alleviates anemia in a mouse model of chronic anemia. Finally, both in control and corticosteroid-treated BFU-E cells PPARα co-occupies many chromatin sites with GR; when activated by PPARα agonists, additional PPARα is recruited to GR-adjacent sites and presumably facilitates GR-dependent BFU-E self-renewal. Our discovery of the role of PPARα agonists in stimulating self-renewal of early erythroid progenitor cells suggests that the clinically tested PPARα agonists we used may improve the efficacy of corticosteroids in treating Epo resistant anemias.

Diamond-Blackfan anemia (DBA) is a congenital disorder characterized by the failure of erythroid progenitor differentiation, severely curtailing red blood cell production. Because many DBA patients fail to respond to corticosteroid therapy, there is considerable need for therapeutics for this disorder. Identifying therapeutics for DBA requires circumventing the paucity of primary patient blood stem and progenitor cells. To this end, we adopted a reprogramming strategy to generate expandable hematopoietic progenitor cells from induced pluripotent stem cells (iPSCs) from DBA patients. Reprogrammed DBA progenitors recapitulate defects in erythroid differentiation, which were rescued by gene complementation. Unbiased chemical screens identified SMER28, a small-molecule inducer of autophagy, which enhanced erythropoiesis in a range of in vitro and in vivo models of DBA. SMER28 acted through autophagy factor ATG5 to stimulate erythropoiesis and up-regulate expression of globin genes. These findings present an unbiased drug screen for hematological disease using iPSCs and identify autophagy as a therapeutic pathway in DBA.

SUMMARY GATA factors establish transcriptional networks that control fundamental developmental processes. Whereas the regulator of hematopoiesis GATA-1 is subject to multiple posttranslational modifications, how these modifications influence GATA-1 function at endogenous loci is unknown. We demonstrate that sumoylation of GATA-1 K137 promotes transcriptional activation only at target genes requiring the coregulator Friend of GATA-1 (FOG-1). A mutation of GATA-1 V205G that disrupts FOG-1 binding and K137 mutations yielded similar phenotypes, although sumoylation was FOG-1-independent, and FOG-1 binding did not require sumoylation. Both mutations dysregulated GATA-1 chromatin occupancy at select sites, FOG-1-dependent gene expression, and were rescued by tethering SUMO-1. While FOG-1- and SUMO-1-dependent genes migrated away from the nuclear periphery upon erythroid maturation, FOG-1- and SUMO-1-independent genes persisted at the periphery. These results illustrate a mechanism that controls trans-acting factor function in a locus-specific manner, and differentially regulated members of the target gene ensemble reside in distinct subnuclear compartments.