SATB1 Defines the Developmental Context for Gene Silencing by Xist in Lymphoma and Embryonic Cells

SummaryThe noncoding Xist RNA triggers silencing of one of the two female X chromosomes during X inactivation in mammals. Gene silencing by Xist is restricted to a special developmental context in early embryos and specific hematopoietic precursors. Here, we show that Xist can initiate silencing in a lymphoma model. We identify the special AT-rich binding protein SATB1 as an essential silencing factor. Loss of SATB1 in tumor cells abrogates the silencing function of Xist. In lymphocytes Xist localizes along SATB1-organized chromatin and SATB1 and Xist influence each other's pattern of localization. SATB1 and its homolog SATB2 are expressed during the initiation window for X inactivation in ES cells. Importantly, viral expression of SATB1 or SATB2 enables gene silencing by Xist in embryonic fibroblasts, which normally do not provide an initiation context. Thus, our data establish SATB1 as a crucial silencing factor contributing to the initiation of X inactivation

Polycomb complexes act redundantly to repress genomic repeats and genes

Polycomb complexes establish chromatin modifications for maintaining gene repression and are essential for embryonic development in mice. Here we use pluripotent embryonic stem (ES) cells to demonstrate an unexpected redundancy between Polycomb-repressive complex 1 (PRC1) and PRC2 during the formation of differentiated cells. ES cells lacking the function of either PRC1 or PRC2 can differentiate into cells of the three germ layers, whereas simultaneous loss of PRC1 and PRC2 abrogates differentiation. On the molecular level, the differentiation defect is caused by the derepression of a set of genes that is redundantly repressed by PRC1 and PRC2 in ES cells. Furthermore, we find that genomic repeats are Polycomb targets and show that, in the absence of Polycomb complexes, endogenous murine leukemia virus elements can mobilize. This indicates a contribution of the Polycomb group system to the defense against parasitic DNA, and a potential role of genomic repeats in Polycomb-mediated gene regulation

In vivo mutation of pre-mRNA processing factor 8 (Prpf8) affects transcript splicing, cell survival and myeloid differentiation

Mutated spliceosome components are recurrently being associated with perturbed tissue development and disease pathogenesis. Cephalophonus (cph), is a zebrafish mutant carrying an early premature STOP codon in the spliceosome component Prpf8 (pre-mRNA processing factor 8). Cph initially develops normally, but then develops widespread cell death, especially in neurons, and is embryonic lethal. Cph mutants accumulate aberrantly spliced transcripts retaining both U2- and U12-type introns. Within early haematopoiesis, myeloid differentiation is impaired, suggesting Prpf8 is required for haematopoietic development. Cph provides an animal model for zygotic PRPF8 dysfunction diseases and for evaluating therapeutic interventions. (C) 2013 Federation of European Biochemical Societies. Published by Elsevier B.V. All rights reserved

Developmentally regulated transcription of mammalian telomeres by DNA-dependent RNA polymerase II

2siMammalian telomeres consist of non-coding TTAGGG repeats that are bound by the multi-protein complex 'shelterin', thus protecting chromosome ends from DNA repair mechanisms and degradation. Mammalian telomeric chromatin is enriched for the constitutive heterochromatin marks H3K9me3, H4K20me3 and HP1 (refs 2, 3, 4, 5, 6, 7). Similar to pericentric heterochromatin, telomeric heterochromatin is thought to be fundamental for the maintenance of chromosomal integrity. Here, we report that telomeric repeats are transcribed by DNA-dependent RNA polymerase II, which, in turn, interacts with the TRF1 shelterin protein. Telomeric RNAs (TelRNAs) contain UUAGGG repeats, are polyadenylated and are transcribed from the telomeric C-rich strand. Transcription of mammalian telomeres is regulated by several mechanisms, including developmental status, telomere length, cellular stress, tumour stage and chromatin structure. Using RNA-flourescent in situ hybridization (FISH), we show that TelRNAs are novel structural components of telomeric chromatin. Importantly, we provide evidence that TelRNAs block the activity of telomerase in vitro, suggesting that TelRNAs may regulate telomerase activity at chromosome ends. Our results indicate that TelRNAs are novel components of mammalian telomeres, which are anticipated to be fundamental for understanding telomere biology and telomere-related diseases, such as cancer and ageing.nonemixedSchoeftner, S.; Blasco Maria, A.Schoeftner, Stefan; Blasco Maria, A