DOSAGE COMPENSATION OF THE ACTIVE X CHROMOSOME IN MAMMALS

描述（由申请人提供）：哺乳动物女性有两条X染色体，男性只有一条。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连锁基因表达不足。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，它使男性和女性的活跃X染色体的总体转录输出增加一倍，以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倍体不足的有害影响。这与果蝇的情况类似，雄性X基因被上调。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。本研究的目的是确定哺乳动物X上调的分子机制。我们推测X的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。我们的目标是(1)确定在发育过程中何时何地建立X上调，(2)研究可能与X上调相关的全局表观遗传修饰，包括组蛋白修饰和已知参与果蝇剂量补偿的候选蛋白，(3)研究大脑特定区域X连锁基因高表达的机制。(4)使用小鼠模型进行X上调的功能研究，我们之前已经证明，当氯离子通道基因Clcr bbbb4位于X染色体上时，与常染色体相比，其表达量增加了一倍。我们的研究对理解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，我们的研究结果表明，随着X连锁智力迟钝的流行，大脑特定区域的X表达总体增加。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.