Rnf12/RLIM is dispensable for X-chromosome inactivation in the mouse embryonic epiblast

In female mice, two forms of X chromosome inactivation (XCI) ensure the selective silencing of female sex chromosomes during mouse embryogenesis. Beginning at the four cell stage, imprinted XCI (iXCI) exclusively silences the paternal X (Xp). Later, around implantation, epiblast cells of the ICM that give rise to the embryo reactivate the Xp and undergo a random form of XCI (rXCI)(1,2). Xist, a long non-coding RNA crucial for both forms of XCI, is activated by the ubiquitin ligase Rnf12/RLIM(3-5). While Rnf12/RLIM is required for triggering iXCI in mice, its importance for rXCI has been controversial. Here, we show that Rnf12/RLIM levels are downregulated in embryonic cells undergoing rXCI. Using mouse genetics we demonstrate that female cells lacking Rnf12/RLIM from pre-implantation stages onwards display hallmarks of XCI including Xist clouds and H3K27me3 foci and display full embryogenic potential. These results provide evidence that Rnf12/RLIM is dispensable for rXCI, indicating that in mice an Rnf12/RLIM-independent mechanism activates Xist in the embryo proper.