DOSAGE COMPENSATION OF THE ACTIVE X CHROMOSOME IN MAMMALS

描述(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。这种根本的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连锁基因表达不足。一种众所周知的剂量补偿机制是X失活，它使性别之间的基因表达剂量相等。我们最近获得了另一种形式的剂量补偿的证据，它使男性和女性活性X染色体的全球转录输出翻一番，达到与常染色体相似的表达水平。X上调的一个关键作用是避免单纯性不足的有害影响，这是本提案的重点。这类似于果蝇中雄性X基因上调的情况。我们在几个哺乳动物物种中使用了微阵列分析来证明X的上调在早期胚胎中建立，并在体细胞组织中保持。我们还发现X连锁基因在脑中的表达水平较高。这项研究的目的是确定哺乳动物X基因上调的分子机制。我们推测，X的上调可能是活性X的表观遗传修饰和/或DMA序列的进化修饰增加基因表达的结果。我们的目标是(1)确定X上调在发育过程中建立的时间和地点，(2)研究可能与X上调相关的全球表观遗传修饰，包括组蛋白修饰和已知参与果蝇剂量补偿的候选蛋白质，(3)研究X连锁基因在大脑特定区域高表达的机制，以及(4)利用我们先前已经显示氯通道基因CLcr&GT；4的小鼠模型进行X上调的功能研究，当它位于X上时，与常染色体相比，它的表达翻了一番。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紊乱和智力低下中的作用具有重要意义。这项拟议的研究与X染色体在人类疾病中的作用有关。尤其重要的是，我们的研究发现，随着X连锁智力低下的流行被很好地记录下来，X在大脑特定区域的表达总体上增加了。维持基因表达的平衡对于正常发育至关重要，这可以从常染色体单体导致的染色体不平衡个体中存在的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.