Chromosome Research最新文献

Centromeres have been the focus of extensive research for almost a century, so it may come as a surprise that a consistent definition and nomenclature for these structures remains elusive. In recent times, centromeric chromatin is most frequently defined by the presence of nucleosomes containing the H3 variant CENP-A and is typically synonymous with the site of the inner-kinetochore. However, crucial mammalian centromere proteins including CENP-B and INCENP have well defined distributions that show very little overlap with CENP-A. Additional protein localisations spanning the primary constriction or forming a band below CENP-A chromatin have been reported. Together, these observations suggest a complex and multi-layered chromatin organisation that is not well served by the canonical dichotomy of 'centromeric' and 'pericentromeric' chromatin. Strikingly, this is not a new observation but was made soon after the discovery of CENP proteins, including in a 1991 publication titled 'When is the centromere not a kinetochore?'. Here we revisit this question, which has become more pertinent following technical innovations in long-read sequencing and super-resolution microscopy. We present a model of centromere organisation for monocentromeres that incorporates additional complexity. We then use this model to reconceptualise diverse centromere forms in other eukaryotes including regional centromeres, holocentromeres and centromeres that lack key proteins including CENP-A. In this way, we hope to move towards a unified understanding of centromeric chromatin.

Centromeres are fundamental chromosomal structures that ensure accurate chromosome segregation during cell division. Despite their conserved and essential role in maintaining genomic stability, centromeres are subject to rapid evolutionary change. At the heart of centromere identity is the histone H3 variant CENP-A, an epigenetic mark that defines and propagates active centromeres and is essential for their function. Recent evidence supports a rapid evolution of centromere DNA sequences but also suggests a certain degree of flexibility in CENP-A deposition and propagation. The phenomenon of centromere drift, recently observed in humans, highlights how the dynamic repositioning of CENP-A and associated epigenetic environment over time maintains a regulated equilibrium, ensuring centromere function despite positional variation. Understanding these processes is crucial for unraveling centromere dynamics and their broader implications for genome stability and evolution.

Four decades ago, the discovery of centromere protein-A (CENP-A) marked a pivotal breakthrough in chromosome biology, revealing the epigenetic foundation of centromere identity. CENP-A, a histone H3 variant, directs the formation of the microtubule-binding kinetochore complex, designating the chromosomal site for its assembly and underpins the accurate partitioning of genetic material during cell division. Errors in cell division can give rise to DNA instability and aneuploidy, implicated in human diseases such as cancer. Therefore, discovering the underlying pathways and mechanisms responsible for the formation, regulation and maintenance of the centromere is important to our understanding of genome stability, epigenetic inheritance, and in providing the knowledge to help generate possible treatments and therapeutics. Here, we review various molecular pathways and mechanisms implicated in maintaining centromere identity and highlight some of the key outstanding questions with a focus on the human centromere.

During spermatogenesis, chromatin structure is remodelled by the incorporation of distinct histone variants and associated posttranslational modifications, followed by the almost complete replacement of histones by protamines in sperm. However, the dynamics of the centromere-specific histone H3 variant CENP-A have not yet been elucidated during spermatogenesis in mammals. Here we investigate CENP-A localisation dynamics in cattle (Bos taurus). In bovine testis tissue sections, we quantify CENP-A intensity in key germ cell types; spermatogonia (pre-meiotic), primary spermatocytes (meiotic) and spermatids (post-meiotic). Our quantitation shows that spermatogonia harbour the highest amount of CENP-A compared to all other germ cell types. Spermatids have approximately one quarter the amount of CENP-A of spermatogonia indicating that overall, it is reduced and maintained through the two meiotic divisions. Yet, we also observed some unexpected dynamics. CENP-A is asymmetrically distributed such that undifferentiated spermatogonia harbour more CENP-A that differentiated spermatogonia that enter meiosis. We also noted an increase in CENP-A intensity in primary spermatocytes during meiotic prophase I, which is indicative of centromere assembly at this time. We also confirm the specific maintenance of CENP-A, and the absence of the centromeric DNA binding protein CENP-B, on mature bull sperm nuclei that have completed histone-to-protamine exchange. Finally, we present a model for centromere positioning in mature sperm nuclei and propose that centralised clustering of centromeres may serve a protective function during histone-to-protamine exchange.

The centromere is a region present on every human chromosome that is essential for mediating chromosome segregation and maintaining genome stability. However, despite its fundamental role in the process of cell division, the centromere is constantly subjected to a wide range of stresses that can challenge their integrity, causing breakages and aneuploidy. In this review, we will examine the plethora of stresses that challenge the centromere, its stress response and how cells cope with perturbations originating from the intracellular and extracellular microenvironment in order to preserve centromere function and, overall, cellular fitness.

Faithful chromosome segregation is facilitated by the centromeres, specialized genomic loci, which connect chromosomes to microtubules in every cell cycle by recruiting the kinetochore complex. However, a single conserved code does not govern the formation and maintenance of centromeres, as we begin to realize that enormous diversity exists in molecular mechanisms dictating centromere homeostasis across species. The fungal kingdom is a vast resource to study and appreciate the divergent nature of the conserved phenomenon of chromosome segregation. Studies in the fungal kingdom enable researchers to view the evolution of centromeres at the molecular level. While some organisms, such as Saccharomyces cerevisiae, rely on a strict genetically determined centromere establishment, most fungi adopt epigenetically driven mechanisms of centromere propagation. This epigenomic regulation ranges from modifications on the underlying DNA to histones forming the centric and pericentric regions. The centromere DNA sequence, arrangement of sequence elements, its transcription state, and the replication timing, as well as its spatial position in the nucleus, play a major role in determining centromere stability and its function. In this review, we aim to highlight the spectrum of centromere regulatory mechanisms observed in fungi and discuss the gaps in the research that can provide new perspectives on centromere biology.

The identification of CENPA, CENPB, and CENPC by Earnshaw and Rothfield 40 years ago has revealed the remarkable diversity and complexity of centromeres and confirmed most seed plants and animals have centromeres comprised of complex satellite arrays. The rapid evolution of centromeres and positive selection on CENPA and CENPC led to the centromere drive model, in which competition between tandem satellite arrays of differing size and centromere strength for inclusion in the egg of animals or megaspore of seed plants during female meiosis drives rapid evolution of centromeres and kinetochore proteins. Here we review recent work showing that non-B-form DNA structures in satellite centromeres make them sites of frequent replication fork stalling, and that repair of collapsed forks by break-induced replication rather than unequal sister chromatid exchange is likely the primary mode of satellite expansion and contraction, providing the variation in satellite copy number that is the raw material of centromere drive. Centromere breaks at replication, rather than errors at mitosis, can account for most centromere misdivisions that underlie aneuploidies in cancer.

Circular minichromosomes could be useful tools for plant biotechnology, yet their long-term structural stability, heritability, and effects on phenotype remain poorly understood. In this study, we report a multi-generational analysis of the Arabidopsis mini1a ring minichromosome, which originated from the chromosome 1 centromere in a haploid induction cross. Is mini1a unstable, as suggested by classical studies on other ring chromosomes? Using whole-genome sequencing of individuals carrying mini1a representing multiple successive generations, we uncovered a major catastrophe driven by DNA breaks and novel junction formation, resulting in a new version of mini1a, that carries a 1.3 Mb deletion in the centromeric region (mini1aΔ). We identified 20 new breakpoints, of which 7 disrupted gene bodies-a frequency unlikely to occur by chance. Interestingly, both mini1a and mini1aΔ could exist in one or two copies and could co-exist in a single plant. Although they were inherited efficiently, their presence was sometimes associated with plant sectors with 100% sterility. These findings highlight the structural plasticity of mini1a. At the same time, they raise questions regarding the mechanisms underlying the observed reduced plant fertility. In summary, circular minichromosomes can be deleterious and biotechnology applications based on the manipulation of minichromosomes will require careful planning and testing.

Centromeres provide the chromosomal scaffold for the assembly of the kinetochore complex, thereby linking replicated sister chromatids to the mitotic spindle, driving their segregation into nascent daughter cells. The location and maintenance of centromeres rely, in large part, on a unique conserved chromatin domain, defined by nucleosomes containing the histone H3 variant, Centromere Protein A (CENP-A), whose discovery 40 years ago we now celebrate. Current models place CENP-A, along with many of its orthologs, at the centre of a self-propagating epigenetic feedback loop that heritably maintains centromere position through mitotic and meiotic divisions. CENP-A is stably recycled through DNA replication but requires replenishment each cell cycle. In many organisms, assembly is restricted to G1 phase, indicating tight cell cycle control of the assembly machinery. Here, we provide a historical overview of the discoveries that led to current models of cell cycle control of centromere assembly, starting with early models of regulation to the intricate, multi-layered phosphoregulation revealed to date. Our review focuses primarily on the human and other animal systems, in which the current view is that negative and positive control through cyclin-dependent kinases and Polo-like kinase 1 combine to link CENP-A assembly to mitotic exit. Cell cycle-coupled CENP-A assembly has been attributed to so-called licensing or priming events. We discuss the validity of these models and terminology and highlight key outstanding questions that remain unanswered.

Eukaryotic chromosome segregation requires spindle microtubules to attach to chromosomes through kinetochores. The chromosomal locus that mediates kinetochore assembly is the centromere and is epigenetically specified in most organisms by a centromeric histone H3 variant called CENP-A. An exception to this is budding yeast, which have short, sequenced-defined point centromeres. In S. cerevisiae, a single CENP-A nucleosome is formed at the centromere and is sufficient for kinetochore assembly. The thermophilic budding yeast Kluyveromyces marxianus also has a point centromere, but its length is nearly double the S. cerevisiae centromere and the number of centromeric nucleosomes and kinetochore attachment sites is unknown. Purification of native kinetochores from K. marxianus yielded a mixed population, with one subpopulation that appeared to consist of doublets, making it unclear whether K. marxianus shares the same attachment architecture as S. cerevisiae. Here, we demonstrate that though the doublet kinetochores have a functional impact on kinetochore strength, kinetochore localization throughout the cell cycle appears conserved between these two yeasts. In addition, whole spindle electron tomography demonstrates that a single microtubule binds to each chromosome. Single-molecule nucleosome mapping analysis suggests the presence of a single centromeric nucleosome. Taken together, we propose that the K. marxianus point centromere assembles a single centromeric nucleosome that mediates attachment to one microtubule.