X染色体失活机理研究

X染色体失活是哺乳动物中为实现雌性XX个体和雄性XY个体间X染色体上基因剂量补偿作用(dosage compensation)而普遍存在的一种现象,表现为雌性个体两条X染色体中的一条结构异固缩和大范围的基因失活。由于失活基因高度甲基化,曾经认为甲基化在这一过程中发挥重要作用并据此提出一些模型,但相反的证据不断积累使人们对甲基化在这一过程中的主导作用发生怀疑。由于X     

RNA与X染色体失活的后成调节

生物体内,由于性别的差异,两性细胞中的Ｘ染色体数目不同。为了平衡具有不同Ｘ染色体数目的细胞中Ｘ连锁基因的表达量,生物体在进化过程中引入了剂量补偿机制（ｄｏｓａｇｅｃｏｍｐｅｎｓａｔｉｏｎ）。在线虫中,两性体的染色体浓缩和分离与一组蛋白复合体有关,这组...     

孤雌胚胎干细胞基因印迹及X染色体失活状态初步研究

目的:研究孤雌胚胎干细胞(phESC)与受精卵来源胚胎干细胞(hESC)在印迹基因表达、X染色体失活等方面的异同。方法:运用实时荧光相对定量PCR、甲基化特异性PCR和免疫荧光染色等方法检测phESC与hESC在父系印迹基因IGF2R,母系印迹基因SNRPN,IGF2相对表达量及X染色体失活状态。结果:①母系印迹基因SNRPN,IGF2在phESC细胞中不表达,而父系印迹基因IGF2R表达量则相对于hESC有近2倍的上调;②XIST基因在第35代phESC细胞中没有表达,意味着早期的phESC没有进行X染色体失活,而到了第55代,XIST基因开始表达并随着分化时间的延长表达量逐渐上调;③XIST启动子甲基化状态及组蛋白H3赖氨酸27三甲基化免疫荧光染色阳性证实phESC在长期培养后启动了X染色体失活。结论:phESC的X染色体失活状态在培养过程中存在不稳定的情况,建议对phESC进行更深入的表观遗传稳定性研究,以确保这种细胞未来安全、高效的应用。     

X染色体失活——一种特殊方式的基因调控

剂量补偿是使Ｘ连锁基因的表达水平在两性间达到平衡的过程。生物界实现剂量补偿的策略有很多种,真兽亚纲哺乳动物是随机失活雌性的一条Ｘ染色体。Ｘ失活开始于ＸＩＣ,然后传播到整条染色体。ＸＩＳＴ基因定位于ＸＩＣ中,参与Ｘ失活的启动,可能是Ｘ失活决定基因。最近在人和小鼠中发现了逃避Ｘ失活的基因。探讨这些基因逃避Ｘ失活的机制有助于理解Ｘ染色体失活是如何对基因表达进行调控的。人和小鼠中有一些基因的Ｘ失活状态不同,提示了性染色体的持续不断的进化改变 。     

5-azaC对人体成纤维细胞失活X染色体DNA复制带型的影响

本文用胞苷的类似物5-氮胞苷(5-azaC)处理两种人体皮肤成纤维细胞——46,XX和46,Xt(X;1)。经过一定时间培养,观察用5-azaC处理人体成纤维细胞中失活X染色体复制带的变化。发现5-azac对迟复制X(LX)和t(X;1)染色体上有一条或多条复制带的染色增深,表示复制提前,大约比对照组提前复制1小时左右。同时发现t(X;1)染色体上,受X失活中心的失活扩散影响的1-10区段,用5-azac处理之后,也有复制提前。复制提前这一现象,从另一方面支持了DNA甲基化,可能是人类X染色体失活的一种机理。     

失活X染色体基因逃逸与系统性红斑狼疮的性别二态性

X染色体失活可平衡女性中两条X染色体的基因剂量。越来越多的证据表明,失活X染色体上存在许多能够逃逸失活的基因。逃逸的机制涉及到DNA、RNA、组蛋白的表观修饰以及众多的调控蛋白和染色质的空间结构。失活X染色体基因逃逸的研究为人类疾病(特别是自身免疫性疾病)性别二态性的研究开辟了新的途径。目前已证实包括TLR7、CD40L、IRAK-1、CXCR3、CXorf21等失活X染色体基因逃逸是系统性红斑狼疮(systemic lupus erythematosus,SLE)女性好发的重要原因。本文主要综述了失活X染色体上基因逃逸以及与SLE性别二态性形成的分子机制。阐明SLE性别二态性形成的分子机制,不仅对疾病的诊断、治疗具有重要意义,而且对深入揭示人类免疫系统的发育及调控机理也有重要的理论意义。     

一个蚕豆病遗传家系G6PD基因突变及X染色体失活偏移检测

对一蚕豆病遗传家系的G6PD基因突变进行分析,检测突变后G6PD酶活变化,并对先证者家系进行X染色体失活（XCI）偏移模式检测,从而预测G6PD突变女性携带者患蚕豆病的风险。取家系成员的外周血样,并提取基因组DNA,用聚合酶链式反应（PCR）和DNA测序法进行序列分析,确定先证者突变位点和突变类型及家庭成员遗传情况,若先证者的母亲和姐姐为G6PD突变携带者,则对先证者母亲和姐姐进行X染色体偏移检测以及酶活检测分析,以评估携带者患蚕豆病的风险,同时对研究对象进行随访。结果患者X染色体上G6PD基因发生点突变c.1376G>T;酶活性检测结果显示该突变使G6PD酶活性下降大约25%,导致蚕豆病发生。该家系的两位女性携带者X染色体失活偏移<80%,未来发生蚕豆病的可能性低。     

哺乳动物性别决定机制的研究进展

哺乳动物的性别决定包括初级性别决定和次级性别决定,是以SRY基因为主导,其他多个基因参与的级联调控过程。近年的研究表明。SRY、DAX1、SOX3等性染色体基因和SOX9、MIS、WT1、SF1等常染色体基因都参与性别决定的级联过程。结合中学生物学教材及发育生物学相关原理,从性染色体上和常染色体上与性别决定有关的基因阐述哺乳动物的性别决定机制,并简述了哺乳动物的性别决定模型。     

雌性畸胎瘤细胞离体分化时X染色体的行为

用雌性畸胎瘤LT细胞(具有两个X染色体)为材料,在离体条件下诱导分化。通过对X连锁的HPRT和G6PD等酶的定量分析,并与Pcc3/A/1畸胎瘤细胞(XO型)对比。结果表明,HPRT与G6PD酶比活性在分化后的LT细胞中,以及在已分化的胚胎体重新种植并传代后的细胞中,均与Pcc3/A/1(XO型)细胞相似,比未分化的细胞降低了一半左右。这些结果可认为,在雌性畸胎瘤细胞离体分化过程中,发生了X染色体的生化分化。     

Mammalian X Chromosome Dosage Compensation: Perspectives From the Germ Line

Sex chromosomes are advantageous to mammals, allowing them to adopt a genetic rather than environmental sex determination system. However, sex chromosome evolution also carries a burden, because it results in an imbalance in gene dosage between females (XX) and males (XY). This imbalance is resolved by X dosage compensation, which comprises both X chromosome inactivation and X chromosome upregulation. X dosage compensation has been well characterized in the soma, but not in the germ line. Germ cells face a special challenge, because genome wide reprogramming erases epigenetic marks responsible for maintaining the X dosage compensated state. Here we explain how evolution has influenced the gene content and germ line specialization of the mammalian sex chromosomes. We discuss new research uncovering unusual X dosage compensation states in germ cells, which we postulate influence sexual dimorphisms in germ line development and cause infertility in individuals with sex chromosome aneuploidy.     

Molecular Features and Functional Constraints in the Evolution of the Mammalian X Chromosome

ABSTRACT

Recent advances in genomic sequencing of multiple organisms have fostered significant advances in our understanding of the evolution of the sex chromosomes. The integration of this newly available sequence information with functional data has facilitated a considerable refinement of our conceptual framework of the forces driving this evolution. Here we address multiple functional constraints that were encountered in the evolution of the X chromosome and the impact that this evolutionary history has had on its modern behavior.     

Evidence for Local Regulatory Control of Escape from Imprinted X Chromosome Inactivation

X chromosome inactivation (XCI) is an epigenetic process that almost completely inactivates one of two X chromosomes in somatic cells of mammalian females. A few genes are known to escape XCI and the mechanism for this escape remains unclear. Here, using mouse trophoblast stem (TS) cells, we address whether particular chromosomal interactions facilitate escape from imprinted XCI. We demonstrate that promoters of genes escaping XCI do not congregate to any particular region of the genome in TS cells. Further, the escape status of a gene was uncorrelated with the types of genomic features and gene activity located in contacted regions. Our results suggest that genes escaping imprinted XCI do so by using the same regulatory sequences as their expressed alleles on the active X chromosome. We suggest a model where regulatory control of escape from imprinted XCI is mediated by genomic elements located in close linear proximity to escaping genes.     

De novo DNA methylation independent establishment of maternal imprint on X chromosome in mouse oocytes

In female mouse embryos, the paternal X chromosome (Xp) is preferentially inactivated during preimplantation development and trophoblast differentiation. This imprinted X-chromosome inactivation (XCI) is partly due to an activating imprint on the maternal X chromosome (Xm), which is set during oocyte growth. However, the nature of this imprint is unknown. DNA methylation is one candidate, and therefore we examined whether disruptions of the two de novo DNA methyltransferases in growing oocytes affect imprinted XCI. We found that accumulation of histone H3 lysine-27 trimethylation, a hallmark of XCI, occurs normally on the Xp, and not on the Xm, in female blastocysts developed from the mutant oocytes. Furthermore, the allelic expression patterns of X-linked genes including Xist and Tsix were unchanged in preimplantation embryos and also in the trophoblast. These results show that a maternal disruption of the DNA methyltransferases has no effect on imprinted XCI and argue that de novo DNA methylation is dispensable for Xm imprinting. This underscores the difference between imprinted XCI and autosomal imprinting.     

Meiotic sex chromosome inactivation in the marsupial Monodelphis domestica

In eutherian mammals, the X and Y chromosomes undergo meiotic sex chromosome inactivation (MSCI) during spermatogenesis in males. However, following fertilization, both the paternally (Xp) and maternally (Xm) inherited X chromosomes are active in the inner cell mass of the female blastocyst, and then random inactivation of one X chromosome occurs in each cell, leading to a mosaic pattern of X-chromosome activity in adult female tissues. In contrast, marsupial females show a nonrandom pattern of X chromosome activity, with repression of the Xp in all somatic tissues. Here, we show that MSCI also occurs during spermatogenesis in marsupials in a manner similar to, but more stable than that in eutherians. These findings support the suggestion that MSCI may have provided the basis for an early dosage compensation mechanism in mammals based solely on gametogenic events, and that random X-chromosome inactivation during embryogenesis may have evolved subsequently in eutherian mammals.     

Genomic Imprinting Leads to Less Selectively Maintained Polymorphism on X Chromosomes

Population-genetic models are developed to investigate the consequences of viability selection at a diallelic X-linked locus subject to genomic imprinting. Under complete paternal-X inactivation, a stable polymorphism is possible under the same conditions as for paternal-autosome inactivation with differential selection on males and females. A necessary but not sufficient condition is that there is sexual conflict, with selection acting in opposite directions in males and females. In contrast, models of complete maternal-X inactivation never admit a stable polymorphism and alleles will either be fixed or lost from the population. Models of complete paternal-X inactivation are more complex than corresponding models of maternal-X inactivation, as inactivation of paternally derived X chromosomes in females screens these chromosomes from selection for a generation. We also demonstrate that polymorphism is possible for incomplete X inactivation, but that the parameter conditions are more restrictive than for complete paternal-X inactivation. Finally, we investigate the effects of recurrent mutation in our models and show that deleterious alleles in mutation–selection balance at imprinted X-linked loci are at frequencies rather similar to those with corresponding selection pressures and mutation rates at unimprinted loci. Overall, our results add to the reasons for expecting less selectively maintained allelic variation on X chromosomes.     

The Implication of Early Chromatin Changes in X Chromosome Inactivation

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Nonrandom X Chromosome Inactivation Is Influenced by Multiple Regions on the Murine X Chromosome

During the development of female mammals, one of the two X chromosomes is inactivated, serving as a dosage-compensation mechanism to equalize the expression of X-linked genes in females and males. While the choice of which X chromosome to inactivate is normally random, X chromosome inactivation can be skewed in F1 hybrid mice, as determined by alleles at the X chromosome controlling element (Xce), a locus defined genetically by Cattanach over 40 years ago. Four Xce alleles have been defined in inbred mice in order of the tendency of the X chromosome to remain active: Xce^a < Xce^b < Xce^c < Xce^d. While the identity of the Xce locus remains unknown, previous efforts to map sequences responsible for the Xce effect in hybrid mice have localized the Xce to candidate regions that overlap the X chromosome inactivation center (Xic), which includes the Xist and Tsix genes. Here, we have intercrossed 129S1/SvImJ, which carries the Xce^a allele, and Mus musculus castaneus EiJ, which carries the Xce^c allele, to generate recombinant lines with single or double recombinant breakpoints near or within the Xce candidate region. In female progeny of 129S1/SvImJ females mated to recombinant males, we have measured the X chromosome inactivation ratio using allele-specific expression assays of genes on the X chromosome. We have identified regions, both proximal and distal to Xist/Tsix, that contribute to the choice of which X chromosome to inactivate, indicating that multiple elements on the X chromosome contribute to the Xce.