Department of Reproductive Medicine and Gynaecology, University Medical Center Utrecht, Utrecht, The Netherlands.
PLoS One. 2012;7(3):e32701. doi: 10.1371/journal.pone.0032701. Epub 2012 Mar 7.
Female mammals inactivate one of their two X-chromosomes to compensate for the difference in gene-dosage with males that have just one X-chromosome. X-chromosome inactivation is initiated by the expression of the non-coding RNA Xist, which coats the X-chromosome in cis and triggers gene silencing. In early mouse development the paternal X-chromosome is initially inactivated in all cells of cleavage stage embryos (imprinted X-inactivation) followed by reactivation of the inactivated paternal X-chromosome exclusively in the epiblast precursors of blastocysts, resulting temporarily in the presence of two active X-chromosomes in this specific lineage. Shortly thereafter, epiblast cells randomly inactivate either the maternal or the paternal X-chromosome. XCI is accompanied by the accumulation of histone 3 lysine 27 trimethylation (H3K27me3) marks on the condensed X-chromosome. It is still poorly understood how XCI is regulated during early human development. Here we have investigated lineage development and the distribution of H3K27me3 foci in human embryos derived from an in-vitro model for human implantation. In this system, embryos are co-cultured on decidualized endometrial stromal cells up to day 8, which allows the culture period to be extended for an additional two days. We demonstrate that after the co-culture period, the inner cell masses have relatively high cell numbers and that the GATA4-positive hypoblast lineage and OCT4-positive epiblast cell lineage in these embryos have segregated. H3K27me3 foci were observed in ∼25% of the trophectoderm cells and in ∼7.5% of the hypoblast cells, but not in epiblast cells. In contrast with day 8 embryos derived from the co-cultures, foci of H3K27me3 were not observed in embryos at day 5 of development derived from regular IVF-cultures. These findings indicate that the dynamics of H3K27me3 accumulation on the X-chromosome in human development is regulated in a lineage specific fashion.
雌性哺乳动物通过失活其两条 X 染色体中的一条来补偿雄性只有一条 X 染色体的基因剂量差异。X 染色体失活由非编码 RNA Xist 的表达启动，该 RNA 在顺式中覆盖 X 染色体并触发基因沉默。在早期小鼠发育中，父本 X 染色体在卵裂期胚胎的所有细胞中最初失活（印迹 X 失活），随后在囊胚的外胚层前体中重新激活失活的父本 X 染色体，仅在外胚层前体中暂时存在两条活性 X 染色体在这个特定的谱系中。此后不久，外胚层细胞随机失活母本或父本 X 染色体。XCI 伴随着凝聚的 X 染色体上组蛋白 3 赖氨酸 27 三甲基化 (H3K27me3) 标记的积累。在人类早期发育过程中，XCI 是如何被调控的，这仍然知之甚少。在这里，我们研究了来自人类着床体外模型的人类胚胎中的谱系发育和 H3K27me3 焦点的分布。在这个系统中，胚胎在经过蜕膜化的子宫内膜基质细胞上共培养至第 8 天，这允许培养期再延长两天。我们证明，在共培养期后，内细胞团具有相对较高的细胞数量，并且这些胚胎中的 GATA4 阳性下胚层谱系和 OCT4 阳性上胚层细胞谱系已经分离。H3K27me3 焦点在约 25%的滋养层细胞和约 7.5%的下胚层细胞中观察到，但在上胚层细胞中没有观察到。与共培养衍生的第 8 天胚胎相比，在常规 IVF 培养衍生的第 5 天胚胎中未观察到 H3K27me3 焦点。这些发现表明，人类发育过程中 X 染色体上 H3K27me3 积累的动力学以谱系特异性的方式受到调控。
