Dosage compensation in the mouse balances up-regulation and silencing of X-linked genes.
Dosage compensation in the mouse balances up-regulation and silencing of X-linked genes.
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小鼠中的剂量补偿平衡了X连锁基因的上调和沉默。
DOI：
10.1371/journal.pbio.0050326
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发表时间：
2007-12
期刊：
影响因子：
9.8
通讯作者：
Turner, Bryan M
中科院分区：
文献类型：
--
作者：
Lin, Hong;Gupta, Vibhor;Vermilyea, Matthew D;Falciani, Francesco;Lee, Jeannie T;O'Neill, Laura P;Turner, Bryan M
哺乳动物中的剂量补偿涉及XX雌性中一条X染色体的沉默，并且需要Xist RNA的顺式表达。待灭活的X在发育的胚泡阶段的内细胞团（ICM）的细胞中随机选择。来自雌性小鼠ICM的胚胎干细胞（ES）具有两条活性X染色体，其中一条随着细胞在培养中分化而失活，为研究X失活的动力学提供了一个强大的模型系统。使用微阵列检测未分化的雌性和雄性小鼠ES细胞中X连锁基因的表达，我们检测到活性X染色体相对于常染色体的表达的全局上调（1.4至1.6倍）。我们在培养的雄性胚泡ICM中也发现了类似的上调。在雄性ES细胞中，在分化2-3周后上调达到2倍，从而平衡单X和二倍体常染色体之间的表达。我们发现，沉默的X-连锁基因在女性胚胎干细胞中发生在一个基因的基因的基础上，在整个分化，与一些基因失活早期，其他人晚，和一些逃避一起。令人惊讶的是，通过在杂交ES细胞中的等位基因特异性分析，我们还鉴定了在未分化细胞中沉默的基因亚组。我们提出，X-连锁基因沉默在女性ES细胞的Xist RNA通过X染色体领土的传播细胞分化，与沉默时间依赖于他们的接近Xist基因座的个别基因。在果蝇和人类这样的生物体中，两性之间存在着主要的染色体差异：雌性有两条大的、基因丰富的X染色体，雄性有一条X染色体和一条小的、基因贫乏的Y染色体。已经发展了各种策略来平衡单个X和常染色体之间以及性别之间的X连锁基因表达（一种称为剂量补偿的现象）。在黑腹果蝇中，来自雄性X的表达上调约2倍，从而平衡X到常染色体和雌性到雄性的表达。相比之下，哺乳动物在需要Xist基因的非翻译RNA产物的过程中沉默两个雌性X中的一个。这平衡了雌性对雄性的表达，但使两性都只有一条功能性X染色体。使用小鼠胚胎干细胞和微阵列表达分析，我们发现小鼠的剂量补偿比以前认为的更复杂，X连锁基因在雄性和雌性细胞中上调，以平衡X对常染色体的表达。随着分化的进行，雌性细胞显示出两个最初活跃的X之一的表达逐渐丧失。令人惊讶的是，沉默发生在基因的基础上超过2-3周的分化;一些基因完全逃脱，而一个亚组的基因，往往邻近Xist基因座，甚至在未分化的细胞中沉默。我们提出，女性X-连锁基因沉默的Xist RNA通过X染色体领土的分化进行渐进传播。在小鼠胚胎干细胞中，X：常染色体表达的平衡是通过上调两种性别的X连锁基因和一个女性X染色体上的基因沉默来实现的。
Dosage compensation in mammals involves silencing of one X chromosome in XX females and requires expression, in cis, of Xist RNA. The X to be inactivated is randomly chosen in cells of the inner cell mass (ICM) at the blastocyst stage of development. Embryonic stem (ES) cells derived from the ICM of female mice have two active X chromosomes, one of which is inactivated as the cells differentiate in culture, providing a powerful model system to study the dynamics of X inactivation. Using microarrays to assay expression of X-linked genes in undifferentiated female and male mouse ES cells, we detect global up-regulation of expression (1.4- to 1.6-fold) from the active X chromosomes, relative to autosomes. We show a similar up-regulation in ICM from male blastocysts grown in culture. In male ES cells, up-regulation reaches 2-fold after 2–3 weeks of differentiation, thereby balancing expression between the single X and the diploid autosomes. We show that silencing of X-linked genes in female ES cells occurs on a gene-by-gene basis throughout differentiation, with some genes inactivating early, others late, and some escaping altogether. Surprisingly, by allele-specific analysis in hybrid ES cells, we also identified a subgroup of genes that are silenced in undifferentiated cells. We propose that X-linked genes are silenced in female ES cells by spreading of Xist RNA through the X chromosome territory as the cells differentiate, with silencing times for individual genes dependent on their proximity to the Xist locus. In organisms such as fruit flies and humans, major chromosomal differences exist between the sexes: females have two large, gene-rich X chromosomes, and males have one X and one small, gene-poor Y. Various strategies have evolved to balance X-linked gene expression between the single X and the autosomes, and between the sexes (a phenomenon called dosage compensation). In Drosophila melanogaster, expression from the male X is up-regulated approximately 2-fold, thereby balancing both X-to-autosome and female-to-male expression. In contrast, mammals silence one of the two female Xs in a process requiring the untranslated RNA product of the Xist gene. This balances female-to-male expression but leaves both sexes with only one functional X chromosome. Using mouse embryonic stem cells and microarray expression analysis, we found that dosage compensation in mice is more complex than previously thought, with X-linked genes up-regulated in both male and female cells so as to balance X-to-autosome expression. As differentiation proceeds, female cells show progressive loss of expression from one of the two initially active Xs. Surprisingly, silencing occurs on a gene-by-gene basis over 2–3 week of differentiation; some genes escape altogether, whereas a subgroup of genes, often adjacent to the Xist locus, is silenced even in undifferentiated cells. We propose that female X-linked genes are silenced by progressive spreading of Xist RNA through the X chromosome territory as differentiation proceeds. In mouse embryonic stem cells, X:autosome expression balance is achieved by up-regulating X-linked genes in both sexes and gene-by-gene silencing on one female X chromosome.