Master Regulation of Centromere Function by the Highly Conserved Mis18 Complex

我们体内的细胞每次分裂时都会复制它们的基因组。为了确保细胞分裂过程没有任何故障地进行到完成，在亲本细胞中复制后包装在染色体中的基因组DNA必须在两个子细胞之间均匀而准确地分离。这意味着每次细胞分裂时，新形成的子代细胞将继承与亲代细胞最初拥有的相同数量的遗传物质。被称为“着丝粒”的特殊结构聚集在染色体上被称为“着丝粒”的特定位置，以确保染色体分离到子细胞中总是准确和没有错误的。着丝粒对基因组的稳定性至关重要，因为聚集在着丝粒上的动粒必须附着在称为微管的线状结构上，微管在细胞分裂期间会将每个复制的染色体拉向一个且只有一个子细胞。着丝粒功能障碍和随之而来的着丝粒缺陷可导致染色体错误分离，这在各种癌症和遗传疾病中都是常见的，如唐氏综合症。在大多数生物中，着丝粒-微管附着只发生在每条染色体上的一个位置。因此，每条染色体上必须只有一个着丝粒。虽然细胞中的大部分染色体DNA紧紧地包裹在常规的组蛋白周围，但着丝粒DNA的独特之处在于它包裹着一种特殊的着丝粒特异性组蛋白CENP-A。是CENP-A，而不是潜在的DNA序列本身，指定了着丝粒在每条染色体上的组装位置。已知CENP-A对着丝粒功能和着丝粒形成是必不可少的：该蛋白的突变已被证明对染色体分离产生不利影响。CENP-A如何识别着丝粒DNA并标记功能着丝粒一直是一个谜，CENP-A在着丝粒上的组装依赖于高度保守的着丝粒蛋白Mis18和CENP-A特异的伴侣蛋白HJURP。调节CENP-A装载时间的事件序列首先由Mis18启动，它招募HJURP，然后将CENP-A沉积在着丝粒上。因此，Mis18主控着丝粒上的CENP-A组装，是着丝粒功能所需的最基本的因素。Mis18于2004年首次在分裂酵母中被发现，它是一种基因，当突变时会导致染色体错误分离。随后在人类细胞中的研究表明，Mis18的功能是高度保守的。不足为奇的是，已经发现Mis18在多种人类癌症中存在错误调控。然而，Mis18最初是如何识别着丝粒从而影响CENP-A组装的，目前尚不清楚。本申请中提出的研究主要旨在阐明Mis18及其相关蛋白如何共同调节CENP-A组装和着丝粒功能。从我们的研究中获得的见解将是建立一个更完整的理解细胞如何在细胞分裂期间通过进化过程忠实地分离它们的DNA的关键。

The cells in our body duplicate their genomes every time they divide. In order to make sure that the process of cell division proceeds to completion without any glitches, the genomic DNA packaged in chromosomes following duplication in the parental cell must be equally and accurately segregated between two daughter cells. This means that every time a cell divides, the newly formed daughter cells will inherit the same amount of genetic material that the parental cell initially harboured. Specialised structures known as 'kinetochores' assemble on specific sites on chromosomes known as 'centromeres' to ensure that chromosome segregation into daughter cells is always accurate and error-free. Centromeres are crucial for genome stability, as kinetochores that assemble on them must attach themselves to thread-like structures known as microtubules that will then pull each duplicated chromosome towards one and only one daughter cell during cell division. Centromere dysfunction and consequent kinetochore defects can lead to chromosome mis-segregation, which has been frequently observed in a variety of cancers, and in genetic diseases such as Down's syndrome.In most organisms, kinetochore-microtubule attachment happens at only one site on each individual chromosome. Each chromosome therefore must have only one centromere on it. While much of the chromosomal DNA in cells is tightly wrapped around conventional 'histone' proteins, centromeric DNA is unique in that it is wrapped around a specialised centromere-specific histone protein known as CENP-A. It is CENP-A, rather than the underlying DNA sequence itself, that specifies the site of centromere assembly on each chromosome. CENP-A is known to be essential for centromere function and kinetochore formation: mutations in this protein have been shown to adversely affect chromosome segregation. How CENP-A recognizes centromeric DNA and marks functional centromeres, has remained a mystery for long.The assembly of CENP-A at centromeres is dependent on the highly conserved centromere protein Mis18, and the CENP-A specific chaperone protein HJURP. The sequence of events that regulates the timing of CENP-A loading is first initiated by Mis18, which recruits HJURP, that in turn then deposits CENP-A at centromeres. Mis18 thus master-regulates CENP-A assembly at centromeres, and is the most fundamental factor required for centromere function. Mis18 was first identified in fission yeast in 2004, as a gene which when mutated led to chromosome mis-segregation. Subsequent studies in human cells have shown that Mis18 function is highly conserved. Not surprisingly, Mis18 has been found to be mis-regulated in a variety of human cancers. How Mis18 initially recognizes centromeres to subsequently effect CENP-A assembly, however, remains unclear. The research proposed in this application is broadly aimed at elucidating how Mis18 and its associated proteins, together regulate CENP-A assembly and centromere function. The insights gained from our research will be key to building a more complete understanding of how cells faithfully segregate their DNA during cell division, through the course of evolution.