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顺利完成，在亲本细胞复制后，染色体中包装的基因组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，新形成的子细胞将继承与亲本细胞最初所拥有的相同数量的遗传物质。被称为“着丝点”的特殊结构组装在被称为“着丝粒”的染色体上的特定位置上，以确保染色体分离成子细胞始终准确无误。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着丝点必须附着在被称为微管的线状结构上，然后在细胞分裂过程中，微管会将每条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点缺陷可导致染色体错分离，这在各种癌症和唐氏综合症等遗传疾病中经常观察到。在大多数生物体中，着丝点与微管的附着只发生在每条染色体上的一个位点上。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紧密地包裹在传统的“组蛋白”蛋白上，但着丝粒DNA的独特之处在于，它包裹在一种名为CENP-A的着丝粒特异性组蛋白上。是CENP-A，而不是潜在的DNA序列本身，决定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至关重要：该蛋白的突变已被证明对染色体分离有不利影响。CENP-A如何识别着丝粒DNA并标记功能着丝粒，长期以来一直是一个谜。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特异性伴侣蛋白HJURP。调控CENP-A装载时间的一系列事件首先由Mis18启动，它招募HJURP，然后将CENP-A沉积在着丝粒上。因此，Mis18调控着丝粒上的CENP-A组装，是着丝粒功能所需的最基本因子。Mis18于2004年首次在裂变酵母中被发现，作为一种突变导致染色体错误分离的基因。随后对人类细胞的研究表明，Mis18的功能是高度保守的。不出所料，Mis18在多种人类癌症中被发现存在调控不当。然而，Mis18最初如何识别着丝粒并随后影响CENP-A组装仍不清楚。本应用中提出的研究广泛旨在阐明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过程中忠实地分离它们的DNA，通过进化过程的关键。

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.