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，雄性只有一条。这种根本性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连锁基因表达的缺陷。一个众所周知的剂量补偿机制是X失活，它使两性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，它使男性和女性活跃X染色体的全球转录输出加倍，以达到与常染色体相似的表达水平。X上调的关键作用（本提议的焦点）是避免单倍体不足的有害影响。这与果蝇中雄性X基因上调的情况相似。我们在几种哺乳动物物种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。我们还发现大脑中X连锁基因的表达更高。这项研究的目标是确定哺乳动物X染色体上调的分子机制.我们推测，X的上调可能是由于表观遗传修饰的活性X和/或从DMA序列的进化修饰，以增加基因表达。我们的目的是（1）确定发育过程中X基因上调的时间和位置，（2）研究与X基因上调潜在相关的整体表观遗传修饰，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的高表达机制，以及（4）使用小鼠模型进行X上调的功能研究，在该小鼠模型中，我们先前已经显示当氯通道基因Clcr>4位于X上时，与常染色体相比，其表达加倍。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低下中的作用具有意义。这项研究与X染色体在人类疾病中的作用有关。特别重要的是，我们的研究发现，在大脑的特定区域的整体增加X表达的X连锁精神发育迟滞的患病率是有据可查的。基因表达平衡的维持对于正常发育至关重要，这可以从常染色体单体性导致染色体不平衡的个体中存在多个先天性异常中看出。

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.