Regulation of Gene Expression During Erythropoiesis.

    The molecular events that confer the ability to express lineage-specific genes upon an initially uncommitted, pluripotent hematopoietic stem cell remain a major question in cell differentiation.  Use of an immortalized erythroid cell line as a means to isolate genes that may be important for erythroid function allowed us to identify a novel, erythroid-specific gene, which was named EKLF (erythroid Krüppel-like factor).

    Molecular and biological studies have established that EKLF is an essential component required for globin switching and completion of the definitive erythroid program. Disorders of hemoglobin expression can lead to a variety of hemoglobinopathies, including sickle cell anemia and ß–thalassemia (Cooley’s anemia). As a result, our examination of EKLF’s mechanism of action has illuminated how it regulates the globin locus, and has provided us with a way to reconstruct EKLF so that it can potentially rectify one type of hemoglobin disorder. 

    Our discovery of EKLF has stimulated other investigators around the world to search for analogous genes that can work in a similar fashion to regulate unique targets in other tissues.  EKLF is now the founding member (KLF1) of a family of seventeen proteins, some of which have been directly implicated in suppression of a specific subset of cancers. We are vigorously continuing the study of EKLF (KLF1) using a number of approaches, including biochemical and structure/function analyses of the EKLF protein, identification of its protein partners, and monitoring how EKLF expression itself is so precisely regulated during development.

    Our recent studies show that EKLF becomes acetylated by virtue of its association with a subset of coactivators, leading to enhanced interaction with chromatin remodelers that leads to activated transcription. Surprisingly, EKLF can also associate with corepressors and decrease transcription at selected promoters, suggesting other activities beyond activation of the adult ß-globin gene.  We have most recently found that EKLF is modified by SUMOylation, and that this also controls its repression function.  Quite unexpectedly, we have found that EKLF is expressed in the bipotential progenitor that leads to erythroid cell and may be playing a directive role in that process.  Finally, the BMP4/Smad pathway plays a critical role in transcriptional activation of EKLF in the erythroid cell, likely via highly conserved elements within a relatively small promoter region proximal to its initiation site.

Figure 1. Molecular model of EKLF protein (green ribbon) interacting with DNA (white backbone) through specific amino acid sidechains (yellow). Orange spheres = zinc atoms.

Figure 2. Expression of EKLF during early mammalian development. Brightfield (left) and darkfield (right) photographs of sagittal sections through d7.5 (late headfold stage; top pair) and d14.5 (bottom pair) mouse embryos. Expression of EKLF (white on darkfield background) is initially within the blood islands of the embryonic yolk sac (top pair), which gives rise to primitive erythroid cells. It switches expression later to the fetal liver (bottom pair), which gives rise to the definitive erythroid cell population.

© 2008 Mount Sinai Medical Center