Repositioning of Muscle-specific Genes Relative to the Periphery of SC-35 Domains during Skeletal Myogenesis

Self Cite

194

192

We investigated whether genes escape X chromosome inactivation by positioning outside of the territory defined by XIST RNA. Results reveal an unanticipated higher order organization of genes and noncoding sequences. All 15 X-linked genes, regardless of activity, position on the border of the XIST RNA territory, which resides outside of the DAPI-dense Barr body. Although more strictly delineated on the inactive X chromosome (Xi), all genes localized predominantly to the outer rim of the Xi and active X chromosome. This outer rim is decorated only by X chromosome DNA paints and is excluded from both the XIST RNA and dense DAPI staining. The only DNA found well within the Barr body and XIST RNA territory was centromeric and Cot-1 DNA; hence, the core of the X chromosome essentially excludes genes and is composed primarily of noncoding repeat-rich DNA. Moreover, we show that this core of repetitive sequences is expressed throughout the nucleus yet is silenced throughout Xi, providing direct evidence for chromosomewide regulation of ''junk'' DNA transcription. Collective results suggest that the Barr body, long presumed to be the physical manifestation of silenced genes, is in fact composed of a core of silenced noncoding DNA. Instead of acting at a local gene level, XIST RNA appears to interact with and silence core architectural elements to effectively condense and shut down the Xi.Barr body ͉ chromosome territory ͉ nuclear organization ͉ XIST ͉ noncoding RNA X inactivation in mammalian females is a prominent example of the formation of facultative heterochromatin during early development, which prevents the deleterious effects of overexpression of X-linked genes. In interphase, the inactive X chromosome (Xi) is found as a condensed heterochromatic Barr body, usually positioned at the nuclear or nucleolar periphery (1-4). Because many genes have been identified that escape X inactivation, packaging differences of sequences at some level within the Xi is presumed (5, 6). Although it is generally assumed that the Barr body is condensed DNA comprising genes normally expressed on the active X chromosome (Xa), the specific makeup of the Barr body has not been investigated.X inactivation is a multistep process initiated by XIST (7-9). Just after XIST RNA sweeps across the chromosome, a defined pattern of chromatin changes including histone modifications, recruitment of macroH2A, and methylation occurs (for review see refs. 10 and 11). We have shown that XIST RNA remains in the nucleus, functionally associated with the inactive chromatin, forming an interphase territory coincident with Xi (12, 13). We incorporated our results into two models for the higher-level organization of the inactive X chromosome (12). In each alternate view, XIST RNA would induce differences in the packaging of the chromatin to facilitate inactivation. In the first model, all genes, regardless of whether they escape from or are subject to X inactivation, would be interspersed throughout the chromosome territory and would not be cytologically disti...

Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

The X chromosome is organized into a gene-rich outer rim and an internal core containing silenced nongenic sequences

Clemson

Hall

Byron

et al. 2006

Self Cite

194

192

Section: Inactivation ͉ Y Chromosomementioning

confidence: 99%

“…This level of nuclear reorganization is often accompanied by a visible level of chromatin decondensation (5, 6, 9). The precise function, if any, of repositioning toward the outside of CTs remains unclear (1), but it may allow for genes to access a nuclear environment enriched in the components of the transcription (10) and/or mRNAprocessing machinery (4,11,12) and so enhance the efficiency of transcription.The interplay between this level of higher-order chromatin organization and epigenetic mechanisms that act at primary levels of chromatin structure, such as DNA methylation, is a little-explored area. In plants, changes in the architecture of CTs can be induced by treatment with inhibitors of DNA methylation (13).…”

mentioning

confidence: 99%

Chromosome territory reorganization in a human disease with altered DNA methylation

Matarazzo

Boyle

D’Esposito

et al. 2007

Chromosome territory (CT) organization and chromatin condensation have been linked to gene expression. Although individual genes can be transcribed from inside CTs, some regions that have constitutively high expression or are coordinately activated loop out from CTs and decondense. The relationship between epigenetic marks, such as DNA methylation, and higher-order chromatin structures is largely unexplored. DNMT3B mutations in immunodeficiency centromeric instability facial anomalies (ICF) syndrome result in loss of DNA methylation at particular sites, including CpG islands on the inactive X chromosome (Xi). This allows the specific effects of DNA methylation on CTs to be examined. Using fluorescence in situ hybridization, we reveal a differential organization of the human pseudoautosomal region (PAR)2 between the CTs of the X and Y in normal males and the active X (Xa) and the Xi in females. There is also a more condensed chromatin structure on Xi compared with Xa in this region. PAR2 genes are relocalized toward the outside of the Y and Xi CTs in ICF, and on the Xi, we show that this can extend to genes distant from the site of DNA hypomethylation itself. This reorganization is not simply a reflection of the transcriptional activation of the relocalized genes. This report of altered CT organization in a human genetic disease illustrates that DNA hypomethylation at restricted sites in the genome can lead to more extensive changes in nuclear organization away from the original site of epigenetic change. X inactivation ͉ Y chromosomeT o fully understand the orchestration of gene expression, it is necessary to understand how chromatin is spatially organized within the cell nucleus. In particular, the position of a gene with respect to its chromosome territory (CT) and the chromatin condensation of its genomic region have been linked to gene activation and repression (1). Although individual genes can be transcribed from inside of CTs (2), some regions that have constitutively high gene expression (3,4) or are subject to coordinate gene activation during development (5, 6) or differentiation (7) or in response to physiological stimuli (8) locate at the edge or outside of their CT. This level of nuclear reorganization is often accompanied by a visible level of chromatin decondensation (5, 6, 9). The precise function, if any, of repositioning toward the outside of CTs remains unclear (1), but it may allow for genes to access a nuclear environment enriched in the components of the transcription (10) and/or mRNAprocessing machinery (4,11,12) and so enhance the efficiency of transcription.The interplay between this level of higher-order chromatin organization and epigenetic mechanisms that act at primary levels of chromatin structure, such as DNA methylation, is a little-explored area. In plants, changes in the architecture of CTs can be induced by treatment with inhibitors of DNA methylation (13). However, the genome-wide demethylation that follows 5-azacytidine treatment or genetic deficiency in DNA methyltransferase...

“…The combination of genomic sequences and genome-wide expression studies have revealed another form of nonrandom gene distribution, termed tandem gene arrays (TGAs), which represent contiguous stretches of gene loci that display differential regulation during cellular development and/or function (4,5). Importantly, gene clusters and TGAs also demonstrate expression-dependent nuclear localization patterns (6-9); therefore, gene loci are both deterministically arrayed along chromosomes and positioned within the nucleus (10)(11)(12)(13)(14)(15).…”

mentioning

confidence: 99%

Topologically associated domains enriched for lineage-specific genes reveal expression-dependent nuclear topologies during myogenesis

Neems

Garza-Gongora

Smith

et al. 2016

The linear distribution of genes across chromosomes and the spatial localization of genes within the nucleus are related to their transcriptional regulation. The mechanistic consequences of linear gene order, and how it may relate to the functional output of genome organization, remain to be fully resolved, however. Here we tested the relationship between linear and 3D organization of gene regulation during myogenesis. Our analysis has identified a subset of topologically associated domains (TADs) that are significantly enriched for muscle-specific genes. These lineage-enriched TADs demonstrate an expression-dependent pattern of nuclear organization that influences the positioning of adjacent nonenriched TADs. Therefore, lineageenriched TADs inform cell-specific genome organization during myogenesis. The reduction of allelic spatial distance of one of these domains, which contains Myogenin, correlates with reduced transcriptional variability, identifying a potential role for lineage-specific nuclear topology. Using a fusion-based strategy to decouple mitosis and myotube formation, we demonstrate that the cell-specific topology of syncytial nuclei is dependent on cell division. We propose that the effects of linear and spatial organization of gene loci on gene regulation are linked through TAD architecture, and that mitosis is critical for establishing nuclear topologies during cellular differentiation.cell differentiation | gene regulation | nuclear organization | transcriptional noise | 3D image analysis