X-CHROMOSOME INACTIVATION AND IMPRINTING IN TRANSGENICS

雌性哺乳动物的剂量补偿是通过 在早期发育中两条X染色体中的一条失活。 的 失活事件包括X染色体的大多数基因， 但不包括涉及X-Y配对和重组的小区域。 一旦建立，非活动状态是克隆继承的，并且相当稳定。 除此之外，在女性生殖系中， 失活X染色体是卵子发生的重要组成部分。 分子 启动X失活的基础，失活扩散到所有 但是X染色体的特定区域，以及卵子发生中X染色体的重新激活， 我们对此知之甚少，但染色质构象和DNA 甲基化也有牵连。 现在很明显，也许有百分之十的哺乳动物 基因组功能的不同取决于它是否遗传 无论是父亲还是母亲 建立这种差异的过程 基于亲本起源的功能被称为印记。 一些 人类基因显然是受印记，和一些疾病， 由于亲本来源，它们的表达在表型上不同。 对人类脆性X连锁智力低下综合征的研究表明， 突变可能涉及女性生殖系的印记;错误 在正常的X染色体失活-再激活周期中。 印迹已在常染色体的分子水平上得到证实， 小鼠的基因和转基因，似乎涉及差异 甲基化在某些情况下 特定X连锁基因的印记 尚未在分子水平上描述;这种现象必须是 为了理解可能是 负责表达人类脆性X-智力低下 综合征 X染色体失活与印迹的研究进展 利用在X染色体上插入外源基因的转基因小鼠， 染色体作为模型系统。 利用这个系统，我们希望（1） 确定X染色体失活的通常相关分子 （甲基化，DNase I抗性）对该过程很重要， 仅与失活相关，（2）确定是否 X-连锁转基因在染色质中表达不同，组织不同， 当从父本或母本遗传时甲基化，和（3） 确定是否有X染色体失活， 卵原细胞的表达不同，染色质或 与活性X上的相比甲基化了。

Dosage compensation in mammalian females is accomplished by the inactivation of one of the two X chromosomes in early development. The inactivation event encompasses most of the genes of the X chromosome, excluding but a small region involved in X-Y pairing and recombination. Once established, the inactive state is clonally inherited and quite stable, except in the female germline in which reactivation of the inactive X chromosome is an essential part of oogenesis. The molecular basis of initiation of X inactivation, spreading of inactivation to all but specific regions of the X, and reactivation of the X in oogenesis is poorly understood, but differences in chromatin conformation and DNA methylation have been implicated. It is now becoming apparent that perhaps ten percent of the mammalian genome functions differently depending on whether it was inherited maternally or paternally. The process of establishing this differential function based on parental origin is termed imprinting. A number of human genes are clearly subject to imprinting, and several diseases are phenotypically different in their expression due to parental origin. Studies of the human fragile X-linked mental retardation syndrome suggest that the mutation may involve imprinting in the female germline; an error in the normal X-chromosome inactivation-reactivation cycle is suspected. Imprinting has been demonstrated at the molecular level in autosomal genes and transgenes of mice, and appears to involve differential methylation in some cases. Imprinting of specific X-linked genes has not yet been described at a molecular level; this phenomenon must be demonstrated in order to understanding the imprinting that may be responsible for expression of the human fragile X-mental retardation syndrome. Our approach to the study of X-chromosome inactivation and imprinting utilizes transgenic mice having a foreign gene inserted on the X chromosome as a model system. Using this system, we hope to (1) determine which of the usual molecular correlates of X inactivation (methylation, DNase I resistance) are important to the process and which are merely correlated with inactivation, (2) to determine whether or not X-linked transgenes are differently expressed, organized in chromatin or methylated when inherited from the male or female parent, and (3) to determine whether or not the X chromosome which was inactivated in oogonial cells is differently expressed, organized in chromatin or methylated compared to the one that was on the active X.