Xist Repeat A contributes to early recruitment of Polycomb complexes during X-chromosome inactivation.
Xist Repeat A contributes to early recruitment of Polycomb complexes during X-chromosome inactivation.
复制标题
Xist Repeat A 有助于在 X 染色体失活过程中早期招募 Polycomb 复合物。
DOI：
10.1016/j.devcel.2021.04.007
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发表时间：
2021
影响因子：
11.8
通讯作者：
Lee,JeannieT
中科院分区：
文献类型：
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作者：
Colognori,David;Sunwoo,Hongjae;Wang,Danni;Wang,Chen-Yu;Lee,JeannieT
Xist RNA介导的Polycomb募集是X染色体失活（XCI）的重要步骤。在分化胚胎干细胞（ESC）和小鼠胚胎成纤维细胞（MEF）时，Xist RNA的重复B元件对于通过与RNA结合蛋白HNRNPK的相互作用在无活性X（Xi）上富集多梳抑制复合物1和2（PRC 1和PRC 2）是至关重要的（Pintacuda et al.，2017; Bousard等人，2019;脑炎诺里等，2019年）。我们最近的研究揭示了XCI的建立、基因沉默和Polycomb募集可以被解析成两个不同的阶段，其中重复序列A启动沉默和募集，而重复序列B起作用以帮助建立并且特别是稳定它们（脑炎诺里（nori）等人，2020年）。一旦建立，XCI不再需要重复A。这些发现是有吸引力的，因为它们调和了XCI期间两个重复序列的作用的相互冲突的说法。然而，在他们的信中，Wei et al.（2021）认为，在没有重复B的情况下，Polycomb招募应严格归因于重复C，而不是重复A。虽然我们的研究留下了重复C的开放贡献，但它们也表明重复B和C本身不能完全解释Polycomb募集。HNRNPK是一种普遍存在的核蛋白，其结合大量富含C的转录物，因此其本身不能解释Polycomb复合物对Xi或任何其他基因组位置的特异性靶向。据信，需要高局部浓度的HNRNPK才能实现Xist的有效Polycomb靶向（Brockdorff，2018），但尚不清楚该模型是否适用于其他HNRNPK相互作用转录物。HNRNPK eCLIP实验已经将重复B和有限程度的重复C鉴定为命中（Cirillo等人，2016年）。Xist重复序列B具有30个拷贝的高亲和力HNRNPK结合位点（紧密间隔的Cs延伸），而重复序列C具有更少且更短的C延伸，间隔更远。值得注意的是，重复C的核心基序在其他哺乳动物（包括人类）的XIST中不扩增（Pintacuda et al.，2017年）。因此，如果特异性Polycomb募集需要高的局部HNRNPK浓度，则在单独的重复序列C（没有重复序列B）上的HNRNPK结合将不太可能超过Polycomb募集的阈值，达到我们观察到的程度（脑炎诺里（nori）等人，2020年）。遗传和染色质背景的其他方面可能也很重要。Wei等人（2021）指出，早期分化的ESC具有更高水平的Polycomb蛋白，这可能使Repeat C的贡献在这个阶段更具影响力。虽然这是可能的，但Pintacuda等人的缺失使大部分重复C保持完整，但消除了早期分化的ESC中Xi上的所有Polycomb信号（Pintacuda等人，2017年）。这一发现表明，即使在早期分化的ESC中，重复C的任何贡献都是微小的。Brockdorff和他的同事还认为，PRC 2对习近平的招募是间接发生的。他们提出重复B和HNRNPK将PRC 1带到Xi，PRC 1然后催化染色质上的H2 AK 119（H2 AK 119 ub 1）的单泛素化，并且染色质上的H2 AK 119 ub 1标记又结合JARID 2（库珀等人，2016年）。由于JARID 2与PRC 2相互作用，PRC 1可以通过H2 AK 119 ub 1-JARID 2中间体间接将PRC 2带到Xi。然而，H2 AK 119 ub 1标记对JARID 2具有差的亲和力（Kd> 1 μM）（库珀等人，2016年）。这种亲和力太低，在没有额外的条件下无法解释特异性。
Xist RNA-mediated Polycomb recruitment is an important step in X chromosome inactivation (XCI). In differentiating embryonic stem cells (ESCs) and mouse embryonic fibroblasts (MEFs), the Repeat B element of Xist RNA is crucial to enrich Polycomb Repressive Complexes 1 and 2 (PRC1 and PRC2) on the inactive X (Xi) via interaction with the RNA-binding protein HNRNPK (Pintacuda et al., 2017; Bousard et al., 2019; Colognori et al., 2019). Our recent study revealed that establishment of XCI, gene silencing, and Polycomb recruitment can be parsed into two distinct phases, with Repeat A initiating silencing and recruitment and Repeat B functioning to help establish and especially to stabilize them (Colognori et al., 2020). Once established, XCI no longer requires Repeat A. These findings are attractive, as they reconcile conflicting claims over the roles of the two repeats during XCI. In their letter, however, Wei et al.(2021) argue that Polycomb recruitment in the absence of Repeat B should be attributed strictly to Repeat C, and not to Repeat A. While our studies leave open contributions from Repeat C, they also demonstrate that Repeats B and C alone cannot fully explain Polycomb recruitment. HNRNPK is a ubiquitous nuclear protein that binds to a large number of C-rich transcripts and therefore by itself cannot explain the specific targeting of Polycomb complexes to the Xi or any other genomic location. It is believed that high local concentrations of HNRNPK are needed to achieve productive Polycomb targeting for Xist (Brockdorff, 2018), but it is unclear if this model applies to other HNRNPK-interacting transcripts. HNRNPK eCLIP experiments have identified Repeat B and, to a limited extent, Repeat C as hits (Cirillo et al., 2016). Xist Repeat B has 30 copies of high-affinity HNRNPK binding sites (closely spaced stretches of Cs), whereas Repeat C has fewer and shorter C stretches spaced farther apart. Notably, Repeat C’s core motif is not amplified in XIST in other mammals, including humans (Pintacuda et al., 2017). Thus, if high local HNRNPK concentrations are required for specific Polycomb recruitment, HNRNPK binding across Repeat C alone (without Repeat B) would be unlikely to cross the threshold for Polycomb recruitment, to the degree we observed (Colognori et al., 2020). Other aspects of genetic and chromatin context could be important. Wei et al.(2021) point out that early differentiating ESCs have higher levels of Polycomb proteins, which might make Repeat C’s contribution more impactful at this stage. While this is possible, the deletion of Pintacuda et al. left most of Repeat C intact yet abolished all Polycomb signals on the Xi in early differentiating ESCs (Pintacuda et al., 2017). This finding suggests that any contribution of Repeat C is minor even in early differentiating ESCs. Brockdorff and colleagues also argue that PRC2’s recruitment to the Xi occurs indirectly. They propose that Repeat B and HNRNPK bring PRC1 to the Xi, PRC1 then catalyzes monoubiquitylation of H2AK119 (H2AK119ub1) on the chromatin, and the H2AK119ub1 mark on chromatin in turn binds JARID2 (Cooper et al., 2016). Because JARID2 interacts with PRC2, PRC1 could then indirectly bring PRC2 to the Xi via the H2AK119ub1-JARID2 intermediary. However, the H2AK119ub1 mark has a poor affinity for JARID2 (Kd> 1 μM)(Cooper et al., 2016). This affinity is too low to account for specificity without additional