Developmental Epigenetics Group, MRC Clinical Sciences Centre, Faculty of Medicine ICSTM, Hammersmith Hospital, Du Cane Road, London, UK.
Epigenetics Chromatin. 2008 Oct 27;1(1):2. doi: 10.1186/1756-8935-1-2.
X chromosome inactivation is the mechanism used in mammals to achieve dosage compensation of X-linked genes in XX females relative to XY males. Chromosome silencing is triggered in cis by expression of the non-coding RNA Xist. As such, correct regulation of the Xist gene promoter is required to establish appropriate X chromosome activity both in males and females. Studies to date have demonstrated co-transcription of an antisense RNA Tsix and low-level sense transcription prior to onset of X inactivation. The balance of sense and antisense RNA is important in determining the probability that a given Xist allele will be expressed, termed the X inactivation choice, when X inactivation commences.
Here we investigate further the mechanism of Xist promoter regulation. We demonstrate that both sense and antisense transcription modulate Xist promoter DNA methylation in undifferentiated embryonic stem (ES) cells, suggesting a possible mechanistic basis for influencing X chromosome choice. Given the involvement of sense and antisense RNAs in promoter methylation, we investigate a possible role for the RNA interference (RNAi) pathway. We show that the Xist promoter is hypomethylated in ES cells deficient for the essential RNAi enzyme Dicer, but that this effect is probably a secondary consequence of reduced levels of de novo DNA methyltransferases in these cells. Consistent with this we find that Dicer-deficient XY and XX embryos show appropriate Xist expression patterns, indicating that Xist gene regulation has not been perturbed.
We conclude that Xist promoter methylation prior to the onset of random X chromosome inactivation is influenced by relative levels of sense and antisense transcription but that this probably occurs independent of the RNAi pathway. We discuss the implications for this data in terms of understanding Xist gene regulation and X chromosome choice in random X chromosome inactivation.
X 染色体失活是哺乳动物在 XX 雌性中实现 X 连锁基因相对于 XY 雄性剂量补偿的机制。染色体沉默由非编码 RNA Xist 的表达在顺式触发。因此，正确调节 Xist 基因启动子对于在雄性和雌性中建立适当的 X 染色体活性是必要的。迄今为止的研究表明，反义 RNA Tsix 的共转录和 X 失活前的低水平有意义转录。在 X 失活开始时，确定给定的 Xist 等位基因是否表达的意义和反义 RNA 的平衡称为 X 失活选择，这对平衡很重要。
在这里，我们进一步研究了 Xist 启动子调节的机制。我们证明，在未分化的胚胎干细胞（ES）中，有意义和反义转录都调节 Xist 启动子 DNA 甲基化，这表明在影响 X 染色体选择方面存在可能的机制基础。鉴于意义和反义 RNA 参与启动子甲基化，我们研究了 RNA 干扰（RNAi）途径的可能作用。我们表明，在必需的 RNAi 酶 Dicer 缺陷的 ES 细胞中，Xist 启动子的甲基化程度较低，但这种效应可能是这些细胞中从头 DNA 甲基转移酶水平降低的次要后果。与这一致，我们发现 Dicer 缺陷的 XY 和 XX 胚胎表现出适当的 Xist 表达模式，表明 Xist 基因调节没有受到干扰。
我们的结论是，在随机 X 染色体失活开始之前，Xist 启动子的甲基化受有意义和反义转录的相对水平影响，但这可能发生在 RNAi 途径之外。我们根据该数据讨论了在随机 X 染色体失活中理解 Xist 基因调节和 X 染色体选择的意义。
