The work by Wang et al makes a solid

The work by Wang et al. makes a solid case for a role of Spt6 in ESC maintenance. The work also sheds light on a new aspect of Spt6 function at enhancers, connecting Spt6’s function at enhancers to PRC2 and suggesting an antagonistic role in PRC2 assembly. These findings raise some fundamental questions. For instance, what provides the specificity for Spt6 recruitment at OSN-bound SEs? In other words, what makes Spt6 bind OSN-bound SEs, but not OSN-bound repressed elements? Something beyond the presence of OSN has to provide the specificity. This may include the presence of additional TFs, some specific chromatin environment, or perhaps the presence of non-coding transcription at SEs. This last possibility is appealing since Spt6 is known to interact with elongating RNA polymerase II (Yoh et al., 2007).
Introduction Female mammalian embryogenesis and reproduction critically depend on a process called X chromosome inactivation (XCI), which silences one of the two sex chromosomes to achieve dosage compensation. XCI serves as a paradigm to study the epigenetic regulation, whereby gene expression states are maintained independent of DNA sequence. In mice, an imprinted form of XCI (iXCI) is initiated in embryos at the 4-cell stage, silencing exclusively the paternal X (Xp), and this XCI pattern is maintained in extraembryonic tissues. However, epiblast cells, which give rise to the embryo proper, experience a major epigenetic switch around implantation: these Ac-DEVD-AFC reactivate the Xp and undergo a random form of XCI (rXCI), in which the Xp or the maternal X (Xm) is inactivated in each cell with equal probability (Payer, 2016). Both forms of XCI require the long non-coding Xist RNA, which forms clouds on the inactive X chromosome (Xi) from which it is transcribed, leading to X silencing. The X-linked gene Rlim (also known as Rnf12) has emerged as a critical mediator of Xist activity. Rlim encodes a ubiquitin ligase (E3) (Ostendorff et al., 2002) that is involved in transcriptional regulation (Bach et al., 1999, Gontan et al., 2012, Güngör et al., 2007) and shuttles between the nucleus and the cytoplasm (Jiao et al., 2013). In mice, a maternally transmitted Rlim knockout (KO) allele (Δm) results in early lethality of female embryos in a sex-specific parent-of-origin effect due to a failure to maintain iXCI and Xist clouds (Shin et al., 2010, Wang et al., 2016). In contrast, loss of Rlim in female epiblast cells has minimal effect on the rXCI process. RLIM protein levels are downregulated specifically in epiblast cells of implanting embryos, consistent with the lack of rXCI phenotype in Rlim mutant females (Shin et al., 2014). These data identify Rlim-dependent and Rlim-independent mechanisms of XCI in vivo that separately act in pre-implantation embryos and epiblasts, respectively. However, Rlim is crucial for XCI in female embryonic stem cells (ESCs) differentiated in culture (Barakat et al., 2011, Barakat et al., 2014). To further investigate mechanisms of rXCI, we generated female ESCs with a homozygous RlimKO. We found that these cells undergo XCI in vivo but that XCI in vitro is strongly influenced by culture conditions, including both method of differentiation and O2 levels. Our results demonstrate Rlim-dependent and Rlim-independent pathways for XCI exist in ESCs and, together with published data, profoundly change current models of X dosage compensation.