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The central developmental event in the human (h)αglobin gene cluster is selective silencing of the ζ-globin gene as erythropoiesis shifts from primitive erythroblasts in the embryonic yolk sac to definitive erythroblasts in the fetal liver. Previous studies have demonstrated that full developmental silencing of the hζ-globin gene in transgenic mice requires the proximal 2.1 kb of its 3′-flanking region. In the current report, we localize this silencing activity to a 108 bp segment located 1.2 kb 3′ to the ζ-globin gene. Protein(s) in nuclear extracts from cell lines representing the fetal/adult erythroid stage bind specifically to an NF-κB motif located at this site. In contrast, this binding activity is lacking in the nuclear extract of an embryonic-stage erythroid line expressing ζ-globin. This complex is quantitatively recognized by antisera to the NF-κB p50 and to a lesser extent to p65 subunits. A two-base substitution that disrupts NF-κB site protein binding in vitro also results in the loss of the developmental silencing activity in vivo. The data suggest that NF-κB complex formation is a crucial component of hζ-globin gene silencing. This finding expands the roles of this widely distributed transcriptional complex to include negative regulation in mammalian development. [ABSTRACT FROM AUTHOR]

Programmed genetic rearrangements in lymphocytes require transcription at antigen receptor genes to promote accessibility for initiating double-strand break (DSB) formation critical for DNA recombination and repair. Here, we showed that activated B cells deficient in the PTIP component of the MLL3 (mixed-lineage leukemia 3)—MLL4 complex display impaired trimethylation of histone 3 at lysine 4 (H3K4me3) and transcription initiation of downstream switch regions at the immunoglobulin heavy-chain (lgh) locus, leading to defective immunoglobulin class switching. We also showed that PTIP accumulation at DSBs contributes to class switch recombination (CSR) and genome stability independently of lgh switch transcription. These results demonstrate that PTIP promotes specific chromatin changes that control the accessibility of the lgh locus to CSR and suggest a nonredundant role for the MLL3-MLL4 complex in altering antibody effector function. [ABSTRACT FROM AUTHOR]

Recent studies have demonstrated that the Ten-eleven translocation (Tet) family proteins can enzymatically convert 5-methylcytosine (5mC) to 5-hydroxymethylcytosine (5hmC). While 5mC has been studied extensively, little is known about the distribution and function of 5hmC. Here we present a genome-wide profile of 5hmC in mouse embryonic stem (ES) cells. A combined analysis of global 5hmC distribution and gene expression profile in wild-type and Tet1-depleted ES cells suggests that 5hmC is enriched at both gene bodies of actively transcribed genes and extended promoter regions of Polycomb-repressed developmental regulators. Thus, our study reveals the first genome-wide 5hmC distribution in pluripotent stem cells, and supports its dual function in regulating gene expression. [ABSTRACT FROM AUTHOR]