Xist RNA coating of the inactive-X recruits silencing protein complexes. The Polycomb group is a prominent set of proteins recruited to the inactive-X. The Polycomb group proteins post-translationally modify histones, which are then believed to maintain gene silencing across cell division. Like Xist RNA, Polycomb group proteins and the histone modifications they catalyze accumulate on the inactive-X. In the post-implantation female mouse embryo shown above, the red stain detects enrichment of H3K27me3 on the inactive-X. The H3K27me3 enrichment appears more prominent in the extra-embryonic tissues that undergo imprinted X-inactivation compared to the embryonic compartment that is subject to random X-inactivation. The embryonic epiblast cells are identified due to green fluorescence conferred by the mosaic expression of a Gfp transgene on the paternal X chromosome (Xp-Gfp). The extra-embryonic cells are devoid of green fluorescence because of the silencing of Xp-Gfp transgene due to imprinted X-inactivation of the paternal X chromosome.
We have had a long-standing interest in delineating the precise roles of the Polycomb repressive complex 2 (PRC2) and its catalyzed histone H3K27me3 mark in X-inactivation in both imprinted and random X-inactivation (Kalantry et al., 2006; Kalantry & Magnuson, 2006; Maclary et al., 2017). We have also recently discovered that PRC2 function is responsible for selective inactivation of the paternal-X during imprinted X-inactivation. We found that although the paternal-X is subject to imprinted inactivation, the epigenetic imprint resides on the maternal-X. PRC2 function in the oocyte lays down the H3K27me3 mark on the Xist locus, which prevents its expression after fertilization. The Xist locus on the paternal-X doesn't carry H3K27me3 marking and can thus be expressed. In the absence of the core PRC2 protein EED during oogenesis, H3K27me3 marking of the maternal-X is absent. As a result, the early embryo undergoes random rather than imprinted X-inactivation (Harris et al., 2019). Ongoing and future projects include both in vivo and in vitro approaches to dissect the function of Polycomb complexes both temporally and lineage-specifically in X-inactivation and beyond.