Chem. coordinated epigenetic chromatin modifications. INTRODUCTION Regulatory pathways that control development through temporally specified gene activation and repression mechanisms have been recognized as epigenetic (i.e., heritable changes not involving alterations in the primary DNA code) for decades, although the molecules that elicit those developmental programs through epigenetic means have only been elucidated during the past several years. It is currently widely accepted that metazoan transcription factors (both activators and repressors) elicit their specific transcriptional responses through an enormous variety of cofactor molecules whose major purpose is usually to modulate chromatin structure (8, 31). Many such cofactors have been shown to chemically change histones, transcription factors, and cofactors, as well as DNA, in order to elicit the required transcriptional responses. The -globin locus Sulfasalazine has been extensively studied as a paradigm for epigenetic regulation of lineage-specific and developmentally specific gene expression (29), as well as for its clinical relevance to -globin disorders such as sickle cell disease and -thalassemia. The human -globin locus is composed of – (embryonic), G- and A- (fetal), and – and -globin (adult) genes, which are spatially arranged from 5 to 3 and developmentally expressed in the same order (72). The elucidation of the molecular basis for -globin silencing in the adult stage in particular has been the focus of intense investigation, since it has been observed that coinheritance of genetic conditions that confer elevated -globin synthesis can significantly alleviate the symptoms of -globin disorders (44, 56). Previously, several adult-stage -globin repressors have been identified, such as BCL11A and SOX6 that actually interact with each other to repress the -globin genes (67, 84, 86), as well as Ikaros (42) and GATA1 (20). In addition, KLF1 was recently shown to indirectly repress -globin synthesis through activation Sulfasalazine of the gene (3, 92), whereas Myb (66), FOP (78), and COUP-TFII (1) also repress -globin expression by currently undefined mechanisms. While the precise mechanisms by which any of these factors repress -globin transcription is not yet fully understood, overall, the available evidence suggests that the collaborative action of multiple complex signaling pathways, which are still to be fully elucidated, are required for adult stage -globin gene silencing. We previously identified DRED (direct repeat erythroid definitive) as a putative repressor complex that binds to the direct Sulfasalazine repeat (DR) elements, consensus binding sites for nonsteroidal nuclear receptors, in the – and -globin promoters (76). Subsequently, we purified and characterized DRED as a multiprotein complex Sulfasalazine with a molecular Sulfasalazine mass exceeding 500 kDa made up of a heterodimer of the nuclear receptors TR2 and TR4 (TR2/TR4; in standardized nomenclature, NR2C1 and NR2C2, respectively) that can specifically bind to the DR elements of the human embryonic – and fetal -globin promoters, as well as to the murine embryonic Y- and H1-globin promoters (73) (74). Another nuclear receptor COUP-TFII has also been reported to bind to the – and -globin DR sequences (10). However, the functional significance of COUP-TFII binding has not been clearly Rabbit Polyclonal to PPP4R2 decided. Mutating the DR sequences in the – or -globin promoters borne on a YAC (for yeast artificial chromosome) transgene led to derepression of these genes in definitive erythroid cells of transgenic mice (55, 76). Further studies of mice in which wild-type or dominant-negative TR2 or TR4 was forcibly expressed, as well as analysis of mice bearing germ line null mutations in the TR2 and TR4 genes, have.