ABSTRACT
The onset of random X inactivation in mouse requires the switch from a symmetric to an asymmetric state, where the identities of the future inactive and active X chromosomes are assigned. Here we show that RIF1 and KAP1 are two fundamental factors for the definition of the asymmetry. Our data show that at the onset of mESC differentiation, upregulation of the long non-coding RNA Tsix weakens the symmetric RIF1 association with the Xist promoter, and opens a window of opportunity for a more stable association of KAP1. KAP1 is required to sustain high levels of Tsix, thus reinforcing and propagating the asymmetry, and, as a result, marking the future active X chromosome. Furthermore, we show that RIF1 association with the future inactive X chromosome is essential for Xist upregulation. This double-bookmarking system, based on the mutually exclusive relationships of Tsix and RIF1, and RIF1 and KAP1, thus coordinates the identification of the inactive and active X chromosomes and initiates a self-sustaining loop that transforms an initially stochastic event into a stably inherited asymmetric X chromosome state.
Competing Interest Statement
The authors have declared no competing interest.
Footnotes
New data have been added 1. to describe in more detail the problems during female embryonic development. 2. to characterise at single cell levels the dynamics of Tsix upon Kap1 knock down, showing that KAP1 is required to boost Tsix levels upon differentiation and that this is an event that normally preceeds the choice. 3. to strengthen the data showing that Tsix RNA or transcription affect the association of RIF1 with Xist P2 promoter