DOSAGE COMPENSATION OF THE ACTIVE X CHROMOSOME IN MAMMALS

描述（由申请人提供）：哺乳动物雌性有两条 X 染色体，而雄性只有一条。这种根本差异导致了剂量补偿机制的演变。男性面临的一个重要问题是X连锁基因表达的缺陷。众所周知的剂量补偿机制是 X 失活，它可以平衡两性之间的基因表达剂量。我们最近获得了另一种剂量补偿形式的证据，该补偿使男性和女性活性 X 染色体的整体转录输出加倍，以达到与常染色体相似的表达水平。 X 上调的关键作用是避免单倍体不足的有害影响，这是本提案的重点。这与果蝇中雄性 X 上调的情况类似。我们在几种哺乳动物中使用微阵列分析来证明 X 上调在早期胚胎中建立，并在体细胞组织中维持。我们还发现 X 连锁基因在大脑中表达更高。本研究的目标是确定哺乳动物 X 上调的分子机制。我们推测 X 上调可能是由于活性 X 的表观遗传修饰和/或 DMA 序列的进化修饰以增加基因表达所致。我们的目标是 (1) 确定 X 上调在发育过程中何时何地建立，(2) 研究可能与 X 上调相关的全局表观遗传修饰，包括已知参与果蝇剂量补偿的组蛋白修饰和候选蛋白，(3) 研究 X 连锁基因在大脑特定区域高表达的机制，以及 (4) 使用小鼠模型进行 X 上调功能研究，在该模型中，我们之前已显示氯通道表达加倍基因，当与常染色体相比位于 X 时，Clcr>4。我们的研究对于理解 X 染色体的发育和进化生物学以及 X 连锁基因表达在性染色体疾病和智力迟钝中的作用具有重要意义。拟议的研究与 X 染色体在人类疾病中的作用有关。特别重要的是，我们发现大脑特定区域的 X 表达总体增加，因为 X 连锁智力低下的患病率已得到充分记录。维持基因表达的平衡对于正常发育至关重要，这可以从由于常染色体单体性而导致染色体失衡的个体存在多种先天性异常中看出。

DESCRIPTION (provided by applicant): Mammalian females have two X chromosomes and males have only one. This fundamental difference has lead to the evolution of dosage compensation mechanisms. An important problem that males face is a deficiency in X-linked gene expression. A well-known mechanism of dosage compensation is X inactivation, which equalizes gene expression dosage between the sexes. We recently obtained evidence of another form of dosage compensation, which doubles the global transcriptional output from the active X chromosome in males and females to achieve a similar expression level to that of autosomes. A crucial role of X up- regulation, the focus of the present proposal, is to avoid deleterious effects of haplo-insufficiency. This is similar to the situation in Drosophila where the male X is up-regulated. We used microarray analyses in several mammalian species to demonstrate that X up-regulation is established in early embryos, and is maintained in somatic tissues. We also found higher expression of X-linked genes in brain. The goal of the proposed research is to determine the molecular mechanisms of mammalian X up- regulation. We speculate that X up-regulation may result either from epigenetic modifications of the active X and/or from evolutionary modifications of the DMA sequence to increase gene expression. Our Aims are (1) to determine when and where X up-regulation is established during development, (2) to study global epigenetic modifications potentially associated with X up-regulation, including histone modifications and candidate proteins known to be involved in Drosophila dosage compensation, (3) to investigate the mechanisms of high expression of X-linked genes in specific regions of the brain, and (4) to perform functional studies of X up-regulation using a mouse model in which we have previously shown a doubling of expression from the chloride channel gene, Clcr>4, when it is located on the X compared to an autosome. Our research has implications for understanding the developmental and evolutionary biology of the X chromosome and the role of X-linked gene expression in sex chromosome disorders and mental retardation. The proposed research is relevant to the role of the X chromosome in human diseases. Particularly significant are our findings of overall increased X expression in specific regions of the brain as the prevalence of X-linked mental retardation is well documented. Maintenance of the balance of gene expression is critical for normal development, as can be seen from the presence of multiple congenital abnormalities in individuals with chromosomal imbalance due to autosomal monosomy.