Comparison between the CENP-A and histone H3 structures in nucleosomes

Vertebrate centromere architecture: from chromatin threads to functional structures

Centromeres are chromatin structures specialized in sister chromatid cohesion, kinetochore assembly, and microtubule attachment during chromosome segregation. The regional centromere of vertebrates consists of long regions of highly repetitive sequences occupied by the Histone H3 variant CENP-A, and which are flanked by pericentromeres. The three-dimensional organization of centromeric chromatin is paramount for its functionality and its ability to withstand spindle forces. Alongside CENP-A, key contributors to the folding of this structure include components of the Constitutive Centromere-Associated Network (CCAN), the protein CENP-B, and condensin and cohesin complexes. Despite its importance, the intricate architecture of the regional centromere of vertebrates remains largely unknown. Recent advancements in long-read sequencing, super-resolution and cryo-electron microscopy, and chromosome conformation capture techniques have significantly improved our understanding of this structure at various levels, from the linear arrangement of centromeric sequences and their epigenetic landscape to their higher-order compaction. In this review, we discuss the latest insights on centromere organization and place them in the context of recent findings describing a bipartite higher-order organization of the centromere.

Histone variants shape chromatin states in Arabidopsis

How different intrinsic sequence variations and regulatory modifications of histones combine in nucleosomes remain unclear. To test the importance of histone variants in the organization of chromatin we investigated how histone variants and histone modifications assemble in the Arabidopsis thaliana genome. We showed that a limited number of chromatin states divide euchromatin and heterochromatin into several subdomains. We found that histone variants are as significant as histone modifications in determining the composition of chromatin states. Particularly strong associations were observed between H2A variants and specific combinations of histone modifications. To study the role of H2A variants in organizing chromatin states we determined the role of the chromatin remodeler DECREASED IN DNA METHYLATION (DDM1) in the organization of chromatin states. We showed that the loss of DDM1 prevented the exchange of the histone variant H2A.Z to H2A.W in constitutive heterochromatin, resulting in significant effects on the definition and distribution of chromatin states in and outside of constitutive heterochromatin. We thus propose that dynamic exchanges of histone variants control the organization of histone modifications into chromatin states, acting as molecular landmarks.

Centromere Chromatin Dynamics at a Glance

The centromere is a specialized DNA locus that ensures the faithful segregation of chromosomes during cell division. It does so by directing the assembly of an essential proteinaceous structure called the kinetochore. The centromere identity is primarily epigenetically defined by a nucleosome containing an H3 variant called CENP-A as well as by the interplay of several factors such as differential chromatin organization driven by CENP-A and H2A.Z, centromere-associated proteins, and post-translational modifications. At the centromere, CENP-A is not just a driving force for kinetochore assembly but also modifies the structural and dynamic properties of the centromeric chromatin, resulting in a distinctive chromatin organization. An additional level of regulation of the centromeric chromatin conformation is provided by post-translational modifications of the histones in the CENP-A nucleosomes. Further, H2A.Z is present in the regions flanking the centromere for heterochromatinization. In this review, we focus on the above-mentioned factors to describe how they contribute to the organization of the centromeric chromatin: CENP-A at the core centromere, post-translational modifications that decorate CENP-A, and the variant H2A.Z.

The Sequence Dependent Nanoscale Structure of CENP-A Nucleosomes

CENP-A is a histone variant found in high abundance at the centromere in humans. At the centromere, this histone variant replaces the histone H3 found throughout the bulk chromatin. Additionally, the centromere comprises tandem repeats of α-satellite DNA, which CENP-A nucleosomes assemble upon. However, the effect of the DNA sequence on the nucleosome assembly and centromere formation remains poorly understood. Here, we investigated the structure of nucleosomes assembled with the CENP-A variant using Atomic Force Microscopy. We assembled both CENP-A nucleosomes and H3 nucleosomes on a DNA substrate containing an α-satellite motif and characterized their positioning and wrapping efficiency. We also studied CENP-A nucleosomes on the 601-positioning motif and non-specific DNA to compare their relative positioning and stability. CENP-A nucleosomes assembled on α-satellite DNA did not show any positional preference along the substrate, which is similar to both H3 nucleosomes and CENP-A nucleosomes on non-specific DNA. The range of nucleosome wrapping efficiency was narrower on α-satellite DNA compared with non-specific DNA, suggesting a more stable complex. These findings indicate that DNA sequence and histone composition may be two of many factors required for accurate centromere assembly.

CENP-A Regulation and Cancer

In mammals, CENP-A, a histone H3 variant found in the centromeric chromatin, is critical for faithful chromosome segregation and genome integrity maintenance through cell divisions. Specifically, it has dual functions, enabling to define epigenetically the centromere position and providing the foundation for building up the kinetochore. Regulation of its dynamics of synthesis and deposition ensures to propagate proper centromeres on each chromosome across mitosis and meiosis. However, CENP-A overexpression is a feature identified in many cancers. Importantly, high levels of CENP-A lead to its mislocalization outside the centromere. Recent studies in mammals have begun to uncover how CENP-A overexpression can affect genome integrity, reprogram cell fate and impact 3D nuclear organization in cancer. Here, we summarize the mechanisms that orchestrate CENP-A regulation. Then we review how, beyond its centromeric function, CENP-A overexpression is linked to cancer state in mammalian cells, with a focus on the perturbations that ensue at the level of chromatin organization. Finally, we review the clinical interest for CENP-A in cancer treatment.

CENP-A: A Histone H3 Variant with Key Roles in Centromere Architecture in Healthy and Diseased States

Centromeres are key architectural components of chromosomes. Here, we examine their construction, maintenance, and functionality. Focusing on the mammalian centromere- specific histone H3 variant, CENP-A, we highlight its coevolution with both centromeric DNA and its chaperone, HJURP. We then consider CENP-A de novo deposition and the importance of centromeric DNA recently uncovered with the added value from new ultra-long-read sequencing. We next review how to ensure the maintenance of CENP-A at the centromere throughout the cell cycle. Finally, we discuss the impact of disrupting CENP-A regulation on cancer and cell fate.

Centromeric and ectopic assembly of CENP-A chromatin in health and cancer: old marks and new tracks

The histone H3 variant CENP-A confers epigenetic identity to the centromere and plays crucial roles in the assembly and function of the kinetochore, thus ensuring proper segregation of our chromosomes. CENP-A containing nucleosomes exhibit unique structural specificities and lack the complex profile of gene expression-associated histone posttranslational modifications found in canonical histone H3 and the H3.3 variant. CENP-A mislocalization into noncentromeric regions resulting from its overexpression leads to chromosomal segregation aberrations and genome instability. Overexpression of CENP-A is a feature of many cancers and is associated with malignant progression and poor outcome. The recent years have seen impressive progress in our understanding of the mechanisms that orchestrate CENP-A deposition at native centromeres and ectopic loci. They have witnessed the description of novel, heterotypic CENP-A/H3.3 nucleosome particles and the exploration of the phenotypes associated with the deregulation of CENP-A and its chaperones in tumor cells. Here, we review the structural specificities of CENP-A nucleosomes, the epigenetic features that characterize the centrochromatin and the mechanisms and factors that orchestrate CENP-A deposition at centromeres. We then review our knowledge of CENP-A ectopic distribution, highlighting experimental strategies that have enabled key discoveries. Finally, we discuss the implications of deregulated CENP-A in cancer.

Nanoscale dynamics of centromere nucleosomes and the critical roles of CENP-A

In the absence of a functioning centromere, chromosome segregation becomes aberrant, leading to an increased rate of aneuploidy. The highly specific recognition of centromeres by kinetochores suggests that specific structural characteristics define this region, however, the structural details and mechanism underlying this recognition remains a matter of intense investigation. To address this, high-speed atomic force microscopy was used for direct visualization of the spontaneous dynamics of CENP-A nucleosomes at the sub-second time scale. We report that CENP-A nucleosomes change conformation spontaneously and reversibly, utilizing two major pathways: unwrapping, and looping of the DNA; enabling core transfer between neighboring DNA substrates. Along with these nucleosome dynamics we observed that CENP-A stabilizes the histone core against dissociating to histone subunits upon unwrapping DNA, unique from H3 cores which are only capable of such plasticity in the presence of remodeling factors. These findings have implications for the dynamics and integrity of nucleosomes at the centromere.

Histone H2A variants confer specific properties to nucleosomes and impact on chromatin accessibility

In eukaryotes, variants of core histone H2A are selectively incorporated in distinct functional domains of chromatin and are distinguished by conserved sequences of their C-terminal tail, the L1 loop and the docking domain, suggesting that each variant confers specific properties to the nucleosome. Chromatin of flowering plants contains four types of H2A variants, which biochemical properties have not been characterized. We report that in contrast with animals, in Arabidopsis thaliana H2A variants define only four major types of homotypic nucleosomes containing exclusively H2A, H2A.Z, H2A.X or H2A.W. In vitro assays show that the L1 loop and the docking domain confer distinct stability of the nucleosome. In vivo and in vitro assays suggest that the L1 loop and the docking domain cooperate with the C-terminal tail to regulate chromatin accessibility. Based on these findings we conclude that the type of H2A variant in the nucleosome impacts on its interaction with DNA and propose that H2A variants regulate the dynamics of chromatin accessibility. In plants, the predominance of homotypic nucleosomes with specific physical properties and their specific localization to distinct domains suggest that H2A variants play a dominant role in chromatin dynamics and function.

Posttranslational modifications of CENP-A: marks of distinction

Centromeres are specialized chromosome domain that serve as the site for kinetochore assembly and microtubule attachment during cell division, to ensure proper segregation of chromosomes. In higher eukaryotes, the identity of active centromeres is marked by the presence of CENP-A (centromeric protein-A), a histone H3 variant. CENP-A forms a centromere-specific nucleosome that acts as a foundation for centromere assembly and function. The posttranslational modification (PTM) of histone proteins is a major mechanism regulating the function of chromatin. While a few CENP-A site-specific modifications are shared with histone H3, the majority are specific to CENP-A-containing nucleosomes, indicating that modification of these residues contribute to centromere-specific function. CENP-A undergoes posttranslational modifications including phosphorylation, acetylation, methylation, and ubiquitylation. Work from many laboratories have uncovered the importance of these CENP-A modifications in its deposition at centromeres, protein stability, and recruitment of the CCAN (constitutive centromere-associated network). Here, we discuss the PTMs of CENP-A and their biological relevance.

Direct AFM Visualization of the Nanoscale Dynamics of Biomolecular Complexes

High-speed AFM (HS-AFM) is an advanced technique with numerous applications in biology, particularly in molecular biophysics. Developed as a time-lapse AFM technique for direct imaging fully hydrated biological molecules, HS-AFM is currently capable of visualizing the dynamics of biological molecules and their complexes at a video-data acquisition rate. Spatial resolution at the nanometer level is another important characteristic of HS-AFM. This review focuses on examples of primarily protein-DNA complexes to illustrate the high temporal and spatial resolution capabilities of HS-AFM that have resulted in novel models and/or the functional mechanisms of these biological systems.

Conserved molecular structure of the centromeric histone CENH3 in Secale and its phylogenetic relationships

It has been repeatedly demonstrated that the centromere-specific histone H3 (CENH3), a key component of the centromere, shows considerable variability between species within taxa. We determined the molecular structure and phylogenetic relationships of CENH3 in 11 Secale species and subspecies that possess distinct pollination systems and are adapted to a wide range of abiotic and biotic stresses. The rye (Secale cereale) genome encodes two paralogous CENH3 genes, which differ in intron-exon structure and are transcribed into two main forms of the protein, αCENH3 and βCENH3. These two forms differ in size and amino acid substitutions. In contrast to the reported differences in CENH3 structure between species within other taxa, the main forms of this protein in Secale species and subspecies have a nearly identical structure except some nonsynonymous substitutions. The CENH3 proteins are strictly controlled by genetic factors responsible for purifying selection. A comparison between Hordeum, Secale and Triticum species demonstrates that the structure of CENH3 in the subtribes Hordeinae and Triticinae evolved at different rates. The assumption that reticulate evolution served as a factor stabilizing the structure and evolutionary rate of CENH3 and that this factor was more powerful within Secale and Triticum than in Hordeum, is discussed.

Variations on a nucleosome theme: The structural basis of centromere function

The centromere is a specialized chromosomal structure that dictates kinetochore assembly and, thus, is essential for accurate chromosome segregation. Centromere identity is determined epigenetically by the presence of a centromere-specific histone H3 variant, CENP-A, that replaces canonical H3 in centromeric chromatin. Here, we discuss recent work by Roulland et al. that identifies structural elements of the nucleosome as essential determinants of centromere function. In particular, CENP-A nucleosomes have flexible DNA ends due to the short αN helix of CENP-A. The higher flexibility of the DNA ends of centromeric nucleosomes impairs binding of linker histones H1, while it facilitates binding of other essential centromeric proteins, such as CENP-C, and is required for mitotic fidelity. This work extends previous observations indicating that the differential structural properties of CENP-A nucleosomes are on the basis of its contribution to centromere identity and function. Here, we discuss the implications of this work and the questions arising from it.

Centromeres Drive a Hard Bargain

Centromeres are essential chromosomal structures that mediate the accurate distribution of genetic material during meiotic and mitotic cell divisions. In most organisms, centromeres are epigenetically specified and propagated by nucleosomes containing the centromere-specific H3 variant, centromere protein A (CENP-A). Although centromeres perform a critical and conserved function, CENP-A and the underlying centromeric DNA are rapidly evolving. This paradox has been explained by the centromere drive hypothesis, which proposes that CENP-A is undergoing an evolutionary tug-of-war with selfish centromeric DNA. Here, we review our current understanding of CENP-A evolution in relation to centromere drive and discuss classical and recent advances, including new evidence implicating CENP-A chaperones in this conflict.

Co-evolving CENP-A and CAL1 Domains Mediate Centromeric CENP-A Deposition across Drosophila Species

Centromeres mediate the conserved process of chromosome segregation, yet centromeric DNA and the centromeric histone, CENP-A, are rapidly evolving. The rapid evolution of Drosophila CENP-A loop 1 (L1) is thought to modulate the DNA-binding preferences of CENP-A to counteract centromere drive, the preferential transmission of chromosomes with expanded centromeric satellites. Consistent with this model, CENP-A from Drosophila bipectinata (bip) cannot localize to Drosophila melanogaster (mel) centromeres. We show that this result is due to the inability of the mel CENP-A chaperone, CAL1, to deposit bip CENP-A into chromatin. Co-expression of bip CENP-A and bip CAL1 in mel cells restores centromeric localization, and similar findings apply to other Drosophila species. We identify two co-evolving regions, CENP-A L1 and the CAL1 N terminus, as critical for lineage-specific CENP-A incorporation. Collectively, our data show that the rapid evolution of L1 modulates CAL1-mediated CENP-A assembly, suggesting an alternative mechanism for the suppression of centromere drive.

CENP-A nucleosomes localize to transcription factor hotspots and subtelomeric sites in human cancer cells

Background

The histone H3 variant CENP-A is normally tightly regulated to ensure only one centromere exists per chromosome. Native CENP-A is often found overexpressed in human cancer cells and a range of human tumors. Consequently, CENP-A misregulation is thought to contribute to genome instability in human cancers. However, the consequences of such overexpression have not been directly elucidated in human cancer cells.

Results

To investigate native CENP-A overexpression, we sought to uncover CENP-A-associated defects in human cells. We confirm that CENP-A is innately overexpressed in several colorectal cancer cell lines. In such cells, we report that a subset of structurally distinct CENP-A-containing nucleosomes associate with canonical histone H3, and with the transcription-coupled chaperones ATRX and DAXX. Furthermore, such hybrid CENP-A nucleosomes localize to DNase I hypersensitive and transcription factor binding sites, including at promoters of genes across the human genome. A distinct class of CENP-A hotspots also accumulates at subtelomeric chromosomal locations, including at the 8q24/Myc region long-associated with genomic instability. We show this 8q24 accumulation of CENP-A can also be seen in early stage primary colorectal tumors.

Conclusions

Our data demonstrate that excess CENP-A accumulates at noncentromeric locations in the human cancer genome. These findings suggest that ectopic CENP-A nucleosomes could alter the state of the chromatin fiber, potentially impacting gene regulation and chromosome fragility.

Electronic supplementary material

The online version of this article (doi:10.1186/1756-8935-8-2) contains supplementary material, which is available to authorized users.

Nap1 regulates proper CENP-B binding to nucleosomes

CENP-B is a widely conserved centromeric satellite DNA-binding protein, which specifically binds to a 17-bp DNA sequence known as the CENP-B box. CENP-B functions positively in the de novo assembly of centromeric nucleosomes, containing the centromere-specific histone H3 variant, CENP-A. At the same time, CENP-B also prevents undesired assembly of the CENP-A nucleosome through heterochromatin formation on satellite DNA integrated into ectopic sites. Therefore, improper CENP-B binding to chromosomes could be harmful. However, no CENP-B eviction mechanism has yet been reported. In the present study, we found that human Nap1, an acidic histone chaperone, inhibited the non-specific binding of CENP-B to nucleosomes and apparently stimulated CENP-B binding to its cognate CENP-B box DNA in nucleosomes. In human cells, the CENP-B eviction activity of Nap1 was confirmed in model experiments, in which the CENP-B binding to a human artificial chromosome or an ectopic chromosome locus bearing CENP-B boxes was significantly decreased when Nap1 was tethered near the CENP-B box sequence. In contrast, another acidic histone chaperone, sNASP, did not promote CENP-B eviction in vitro and in vivo and did not stimulate specific CENP-B binding to CENP-A nucleosomes in vitro. We therefore propose a novel mechanism of CENP-B regulation by Nap1.

Structure of the CENP-A nucleosome and its implications for centromeric chromatin architecture

Centromeres are dictated by the epigenetic inheritance of the centromeric nucleosome containing the centromere-specific histone H3 variant, CENP-A. The structure of the CENP-A nucleosome has been considered to be the fundamental architecture of the centromeric chromatin. Controversy exists in the literature regarding the CENP-A nucleosome structures, with octasome, hemisome, compact octasome, hexasome, and tetrasome models being reported. Some of these CENP-A nucleosome models may correspond to transient intermediates for the assembly of the mature CENP-A nucleosome; however, their significances are still unclear. Therefore, the structure of the mature CENP-A nucleosome has been eagerly awaited. We reconstituted the human CENP-A nucleosome with its cognate centromeric DNA fragment, and determined its crystal structure. In this review, we describe the structure and the physical properties of the CENP-A nucleosome, and discuss their implications for centromeric chromatin architecture.