Tethering of the transcriptional activator peptide VP16 to these arrays caused repositioning away from the NL (Tumbar & Belmont, 2001). This was initially suggested by experiments with fluorescently tagged lacO arrays that were integrated in a locus near the NL. This, however, does not rule out that the contacts of genes with the NL are the consequence of a lack of transcriptional activity, and vice versa, that genes detach from the NL in response to their activation. In support of this notion, depletion of lamins can lead to derepression of specific genes (primarily in Drosophila) (Shevelyov et al, 2009 Kohwi et al, 2013 Chen et al, 2014) transfer of human inactive promoters from LADs to a neutral chromatin environment can lead to activation of these promoters (Leemans et al, 2019) and artificial tethering of some genes to the NL can reduce their activity (Finlan et al, 2008 Kumaran & Spector, 2008 Reddy et al, 2008 Dialynas et al, 2010). These observations raise the interesting possibility that the NL helps to establish a repressive environment. When cells differentiate, detachment of genes from the NL often coincides with transcriptional activation, while increased NL interactions correlate with reduced transcription (Peric-Hupkes et al, 2010 Lund et al, 2013 Robson et al, 2016, 2017). Most genes inside LADs have very low transcriptional activity (Guelen et al, 2008 Peric-Hupkes et al, 2010 Leemans et al, 2019). How LAD-NL contacts are regulated is poorly understood. The NL contacts of some LADs are highly consistent between cell types, while other LADs interact in cell-type-specific (facultative) manners with the NL. Mammalian genomes have roughly one thousand of such lamina-associated domains (LADs), which are typically hundreds of kb or even a few Mb in size.
In metazoan cell nuclei, large chromatin domains are associated with the nuclear lamina (NL) (Gonzalez-Sandoval & Gasser, 2016 van Steensel & Belmont, 2017 de Leeuw et al, 2018 Kim et al, 2019 Lochs et al, 2019).
These extensive datasets are a resource for the analysis of LAD rewiring by transcription and reveal a remarkable flexibility of interphase chromosomes. Inactivation of active genes can lead to increased NL contacts. Loss of NL interactions coincides with a switch from late to early replication timing, but the latter can involve longer stretches of DNA. The degree of detachment depends on the expression level and the length of the activated gene. Gene activation inside LADs typically causes NL detachment of the entire transcription unit, but rarely more than 50–100 kb of flanking DNA, even when multiple neighboring genes are activated. We addressed these questions by systematic activation or inactivation of individual genes, followed by detailed genome-wide analysis of NL interactions, replication timing, and transcription patterns. How this process works and how it impacts flanking chromosomal regions are poorly understood. Activation of such genes is often accompanied by repositioning toward the nuclear interior.
Transcriptionally inactive genes are often positioned at the nuclear lamina (NL), as part of large lamina-associated domains (LADs).
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