LINE-1 activity in facultative heterochromatin formation during X chromosome inactivation.

During X chromosome inactivation (XCI), Xist RNA coats and silences one of the two X chromosomes in female cells. Little is known about how XCI spreads across the chromosome, although LINE-1 elements have been proposed to play a role. Here we show that LINEs participate in creating a silent nuclear compartment into which genes become recruited. A subset of young LINE-1 elements, however, is expressed during XCI, rather than being silenced. We demonstrate that such LINE expression requires the specific heterochromatic state induced by Xist. These LINEs often lie within escape-prone regions of the X chromosome, but close to genes that are subject to XCI, and are associated with putative endo-siRNAs. LINEs may thus facilitate XCI at different levels, with silent LINEs participating in assembly of a heterochromatic nuclear compartment induced by Xist, and active LINEs participating in local propagation of XCI into regions that would otherwise be prone to escape. Copyright 2010 Elsevier Inc. All rights reserved.