Dynamic Repositioning of Genes in the Nucleus of Lymphocytes Preparing for Cell Division

Abstract

We show that several transcriptionally inactive genes localize to centromeric heterochromatin in the nucleus of cycling but not quiescent (noncycling) primary B lymphocytes. In quiescent cells, centromeric repositioning of inactive loci was induced after mitogenic stimulation. A dynamic repositioning of selected genes was also observed in developing T cells. Rag and TdT loci were shown to relocate to centromeric domains following heritable gene silencing in primary CD4+8+ thymocytes, but not in a phenotypically similar cell line in which silencing occurred but was not heritable. Collectively, these data indicate that the spatial organization of genes in cycling and noncycling lymphocytes is different and that locus repositioning may be a feature of heritable gene silencing.

Nuclear repositioning marks the selective exclusion of lineage‐inappropriate transcription factor loci during T helper cell differentiation

“…A well-studied example of developmentally regulated gene silencing is the Tdt locus in response to TCR signaling in CD4 + CD8 + double-positive thymocytes. In primary thymocytes, silencing becomes a stable trait, while it remains transient in the CD4 + CD8 + doublepositive thymocyte cell line VL3-3M2 [5]. Transient and heritable Tdt silencing have several features in common: promoter accessibility and primary transcript levels decline within 3 h [27,28], histones at the Tdt promoter become (H3K9) deacetylated within 3-6 h and (H3K9) methylated from 6-8 h [28].…”

“…Transient and heritable Tdt silencing have several features in common: promoter accessibility and primary transcript levels decline within 3 h [27,28], histones at the Tdt promoter become (H3K9) deacetylated within 3-6 h and (H3K9) methylated from 6-8 h [28]. Heritable silencing is distinguished from transient silencing by extensive spreading of H3K9 deacetylation and methylation 5 0 and 3 0 of the Tdt promoter [28], and by the repositioning of Tdt loci towards centromeric domains, which is visible as early as 3 h [5,28]. Similarly, centromeric repositioning of CD4 and CD8 co-receptor loci is predictive of stable gene silencing during the transition from the CD4 + CD8 + double-positive to the CD4 or CD8 single-positive stage of thymocyte development and becomes detectable within hours of TCR engagement (M.M.…”

“…To date, locus repositioning and gene silencing are linked largely by correlation [4,5,9,22,28]. However, mechanistic aspects are beginning to emerge: silencing can result from the juxtaposition of genes to centromeres in cis, as a result of chromosomal translocations or of the centromeric integration of transgenes [29].…”

“…The developmental history of cells is reflected in epigenetic modifications [1], such as DNA (CpG) methylation [2], the packaging of DNA in chromatin, post-translational modifications of chromatin proteins [3], and the spatial partitioning of gene loci into transcriptionally permissive and repressive domains within the nucleus [4][5][6].…”

Heterochromatin?many flavours, common themes

“…(35) Some but not all genes utilise these clusters of inactive chromatin as an additional layer of transcriptional control. (96,(98)(99)(100) Silenced imprinted alleles are also located towards the exterior of a nucleus. (101) Interestingly, PcG proteins are found on the surface of condensed chromatin domains, (102) suggesting that they need only a limited access to heterochromatin factors, most likely because they recruit many PcG-specific factors and only rarely HP1.…”