Artificial generation of centromeres and kinetochores to understand their structure and function

Flexible Attachment and Detachment of Centromeres and Telomeres to and from Chromosomes

Accurate transmission of genomic information across multiple cell divisions and generations, without any losses or errors, is fundamental to all living organisms. To achieve this goal, eukaryotes devised chromosomes. Eukaryotic genomes are represented by multiple linear chromosomes in the nucleus, each carrying a centromere in the middle, a telomere at both ends, and multiple origins of replication along the chromosome arms. Although all three of these DNA elements are indispensable for chromosome function, centromeres and telomeres possess the potential to detach from the original chromosome and attach to new chromosomal positions, as evident from the events of telomere fusion, centromere inactivation, telomere healing, and neocentromere formation. These events seem to occur spontaneously in nature but have not yet been elucidated clearly, because they are relatively infrequent and sometimes detrimental. To address this issue, experimental setups have been developed using model organisms such as yeast. In this article, we review some of the key experiments that provide clues as to the extent to which these paradoxical and elusive features of chromosomally indispensable elements may become valuable in the natural context.

Diverse mechanisms of centromere specification

The centromere performs a universally conserved function, to accurately partition genetic information upon cell division. Yet, centromeres are among the most rapidly evolving regions of the genome and are bound by a varying assortment of centromere-binding factors that are themselves highly divergent at the protein-sequence level. A common thread in most species is the dependence on the centromere-specific histone variant CENP-A for the specification of the centromere site. However, CENP-A is not universally required in all species or cell types, making the identification of a general mechanism for centromere specification challenging. In this review, we examine our current understanding of the mechanisms of centromere specification in CENP-A-dependent and independent systems, focusing primarily on recent work.

Permitted and restricted steps of human kinetochore assembly in mitotic cell extracts

Mitotic kinetochores assemble via the hierarchical recruitment of numerous cytosolic components to the centromere region of each chromosome. However, how these orderly and localized interactions are achieved without spurious macromolecular assemblies forming from soluble kinetochore components in the cell cytosol remains poorly understood. We developed assembly assays to monitor the recruitment of green fluorescent protein-tagged recombinant proteins and native proteins from human cell extracts to inner kinetochore components immobilized on microbeads. In contrast to prior work in yeast and Xenopus egg extracts, we find that human mitotic cell extracts fail to support de novo assembly of microtubule-binding subcomplexes. A subset of interactions, such as those between CENP-A-containing nucleosomes and CENP-C, are permissive under these conditions. However, the subsequent phospho-dependent binding of the Mis12 complex is less efficient, whereas recruitment of the Ndc80 complex is blocked, leading to weak microtubule-binding activity of assembled particles. Using molecular variants of the Ndc80 complex, we show that auto-inhibition of native Ndc80 complex restricts its ability to bind to the CENP-T/W complex, whereas inhibition of the Ndc80 microtubule binding is driven by a different mechanism. Together, our work reveals regulatory mechanisms that guard against the spurious formation of cytosolic microtubule-binding kinetochore particles.

Guarding the Genome: CENP-A-Chromatin in Health and Cancer

Faithful chromosome segregation is essential for the maintenance of genomic integrity and requires functional centromeres. Centromeres are epigenetically defined by the histone H3 variant, centromere protein A (CENP-A). Here we highlight current knowledge regarding CENP-A-containing chromatin structure, specification of centromere identity, regulation of CENP-A deposition and possible contribution to cancer formation and/or progression. CENP-A overexpression is common among many cancers and predicts poor prognosis. Overexpression of CENP-A increases rates of CENP-A deposition ectopically at sites of high histone turnover, occluding CCCTC-binding factor (CTCF) binding. Ectopic CENP-A deposition leads to mitotic defects, centromere dysfunction and chromosomal instability (CIN), a hallmark of cancer. CENP-A overexpression is often accompanied by overexpression of its chaperone Holliday Junction Recognition Protein (HJURP), leading to epigenetic addiction in which increased levels of HJURP and CENP-A become necessary to support rapidly dividing p53 deficient cancer cells. Alterations in CENP-A posttranslational modifications are also linked to chromosome segregation errors and CIN. Collectively, CENP-A is pivotal to genomic stability through centromere maintenance, perturbation of which can lead to tumorigenesis.