Mechanisms of escaping X chromosome inactivation and translation to X-linked disease

摘要 在哺乳动物中，雄性X半合子需要X连锁的剂量补偿 XX女性X染色体失活(XCI)基因因为要么是X 染色体可以沉默，杂合子雌性通常是嵌合体表达 因此比半合子更不容易患X连锁疾病 男性。然而，当XCI偏斜时，疾病外显率随着细胞比例的增加而增加 使功能等位基因沉默的基因。结果，男性和女性都暴露在 与隐藏在X染色体上的单基因疾病的累积负担有关。 然而，不活跃的X染色体(Xi)并不是完全沉默的：特别是一些基因 那些在Y染色体上有平行拷贝的人(X-Y基因对)，不受 剂量补偿和逃逸XCI。特纳综合征(TS，核型45，X)表现为 对这种“逃逸”基因的剂量敏感，并增加了显著的X连锁 心血管和肾脏畸形造成的健康负担，以及 自然终止45，X胎。 XCI领域的大部分研究都集中在牵连和剖析必需基因上 沉默路径，使用鼠标X作为主要模型。相比之下， 一种机制(S)，使逃犯能够从原本被压抑的情况下保持表达 染色体领域几乎没有受到关注，尤其是在人类身上。因此， 逃亡者的身份、血统特异性和调节机制仍未解决。 基于我们的等位基因特异性基因组学和XCI专业知识，我们建议解决这些问题 关于人类多能干细胞45，X/46，XX等基因的问题。这个平台 将使我们能够1。)量化人类X基因及其沿线的等位基因特异性表达 不同的细胞谱系，2)剖析逃犯的本地和长期监管逻辑， 和3.)评估个别逃犯对剂量敏感的贡献。 这些进展将有助于解释人类X连锁变体，补充 扩大了基因调控元件的词汇表，并揭示了逃犯是如何设法 抵制沉默。这种机械的洞察力可能会产生新的操纵手段 调控元素，并随后转化为泛化的表观遗传学方法 杂合子女性的X连锁疾病以及TS的基因剂量校正 以及人类基因组中的其他剂量敏感型疾病。

Abstract In mammals, male X hemizygosity neccesitates dosage compensation of X-linked genes by means of X chromosome inactivation (XCI) in XX females. Because either X chromosome can be silenced, heterozygous females are typically mosaic for expression of parental alleles and therefore less susceptible to X-linked disorders than hemizygous males. When XCI is skewed however, disease penetrance rises with the fraction of cells that have silenced the functional allele. As a result, both males and females are exposed to the cumulative burden of monogenic disease harbored on the X chromosome. Yet, the inactive X chromsome (Xi) is not entirely silent: some genes, in particular those with paralogous copies on the Y chromosome (X-Y gene pairs), are exempt from dosage compensation and escape XCI. Turner syndrome (TS, karyotype 45,X) reflects a sensitivity to the dosage of such “escapee” genes, and adds to the significant X-linked health burden through cardiovascular and renal malformations, as well as high rates of spontaneous termination of 45,X conceptuses. Much of the XCI field has focused on implicating and dissecting requisite gene silencing pathways, using the mouse X as the primary model. In contrast, the mechanism(s) that enable escapees to sustain expression from an otherwise repressed chromosome territory have received little attention, especially in humans. As a result, the identity, lineage-specificity, and regulatory mechanisms of escapees remain unresolved. Building on our allele-specific genomics and XCI expertise, we propose to address these questions in otherwise isogenic 45,X/46,XX human pluripotent stem cells. This platform will enable us to 1.) quantify allele-specific expression across the human X and along distinct cell lineages, 2.) dissect the local and long-range regulatory logic of escapees, and 3.) assess dosage-sensitive contributions of individual escapees. These advances will aid the interpretation of human X-linked variants, complement an expanding glossary of gene regulatory elements, and reveal how escapees manage to resist silencing. Such mechanstic insights may yield novel means of manipulating regulatory elements, and subsequently translate into generalized, epigenetic approaches to X-linked disease in heterozygous females, as well as gene dosage correction in TS and other dosage-sensitive disorders in the human genome.