Cell-cycle-dependent structural transitions in the human CENP-A nucleosome in vivo

High-resolution analysis of human centromeric chromatin

Centromeres are marked by the centromere-specific histone H3 variant CENP-A/CENH3. Throughout the cell cycle, the constitutive centromere-associated network is bound to CENP-A chromatin, but how this protein network modifies CENP-A nucleosome conformations in vivo is unknown. Here, we purify endogenous centromeric chromatin associated with the CENP-C complex across the cell cycle and analyze the structures by single-molecule imaging and biochemical assays. CENP-C complex-bound chromatin was refractory to MNase digestion. The CENP-C complex increased in height throughout the cell cycle culminating in mitosis, and the smaller CENP-C complex corresponds to the dimensions of in vitro reconstituted constitutive centromere-associated network. In addition, we found two distinct CENP-A nucleosomal configurations; the taller variant was associated with the CENP-C complex. Finally, CENP-A mutants partially corrected CENP-C overexpression-induced centromeric transcription and mitotic defects. In all, our data support a working model in which CENP-C is critical in regulating centromere homeostasis by supporting a unique higher order structure of centromeric chromatin and altering the accessibility of the centromeric chromatin fiber for transcriptional machinery.

Centromeres are stress-induced fragile sites

Centromeres are unique loci on eukaryotic chromosomes and are complexed with centromere-specific histone H3 molecules (CENP-A in mammals, Cse4 in yeast). The centromere provides the binding site for the kinetochore that captures microtubules and provides the mechanical linkage required for chromosome segregation. Centromeres encounter fluctuations in force as chromosomes jockey for position on the metaphase spindle. We have developed biological assays to examine the response of centromeres to high force. Torsional stress is induced on covalently closed DNA circles from supercoiling. Plasmid-borne centromeres with single-nucleotide inactivating mutations exhibit a high conversion frequency to plasmid dimer species. Conversion to dimers is dependent on the activity of the Rad1 single-strand endonuclease, indicative of unwinding a region of the centromere sequence in the absence of a functional kinetochore. To determine the region of unwinding, we used conditionally functional dicentric chromosomes to exert tension. Centromere DNA is exquisitely sensitive to cleavage following activation of the dicentric chromosome. Cleavage is dependent on the action of Rad1, highlighting the propensity of centromeres to unwind in response to supercoiling or mechanical stress. These studies provide mechanistic insights into the evolution of AT-rich pericentromere DNA throughout phylogeny and suggest a mechanism for stress-induced error correction at the centromere.

The yeast genome is globally accessible in living cells

Eukaryotic genomes are packaged into chromatin, which is composed of condensed filaments of regularly spaced nucleosomes, resembling beads on a string. The nucleosome contains ~147 bp of DNA wrapped almost twice around a central core histone octamer. The packaging of DNA into chromatin represents a challenge to transcription factors and other proteins requiring access to their binding sites. Consequently, control of DNA accessibility is thought to play a key role in gene regulation. Here we measure DNA accessibility genome wide in living budding yeast cells by inducible expression of DNA methyltransferases. We find that the genome is globally accessible in living cells, unlike in isolated nuclei, where DNA accessibility is severely restricted. Gene bodies are methylated at only slightly slower rates than promoters, indicating that yeast chromatin is highly dynamic in vivo. In contrast, silenced loci and centromeres are strongly protected. Global shifts in nucleosome positions occur in cells as they are depleted of ATP-dependent chromatin remodelers, suggesting that nucleosome dynamics result from competition among these enzymes. We conclude that chromatin is in a state of continuous flux in living cells, but static in nuclei, suggesting that DNA packaging in yeast is not generally repressive.

Native and tagged CENP-A histones are functionally inequivalent

BACKGROUND

Over the past several decades, the use of biochemical and fluorescent tags has elucidated mechanistic and cytological processes that would otherwise be impossible. The challenging nature of certain nuclear proteins includes low abundancy, poor antibody recognition, and transient dynamics. One approach to get around those issues is the addition of a peptide or larger protein tag to the target protein to improve enrichment, purification, and visualization. However, many of these studies were done under the assumption that tagged proteins can fully recapitulate native protein function.

RESULTS

We report that when C-terminally TAP-tagged CENP-A histone variant is introduced, it undergoes altered kinetochore protein binding, differs in post-translational modifications (PTMs), utilizes histone chaperones that differ from that of native CENP-A, and can partially displace native CENP-A in human cells. Additionally, these tagged CENP-A-containing nucleosomes have reduced centromeric incorporation at early G1 phase and poorly associates with linker histone H1.5 compared to native CENP-A nucleosomes.

CONCLUSIONS

These data suggest expressing tagged versions of histone variant CENP-A may result in unexpected utilization of non-native pathways, thereby altering the biological function of the histone variant.

Single molecule analysis of CENP-A chromatin by high-speed atomic force microscopy

Chromatin accessibility is modulated in a variety of ways to create open and closed chromatin states, both of which are critical for eukaryotic gene regulation. At the single molecule level, how accessibility is regulated of the chromatin fiber composed of canonical or variant nucleosomes is a fundamental question in the field. Here, we developed a single-molecule tracking method where we could analyze thousands of canonical H3 and centromeric variant nucleosomes imaged by high-speed atomic force microscopy. This approach allowed us to investigate how changes in nucleosome dynamics in vitro inform us about transcriptional potential in vivo. By high-speed atomic force microscopy, we tracked chromatin dynamics in real time and determined the mean square displacement and diffusion constant for the variant centromeric CENP-A nucleosome. Furthermore, we found that an essential kinetochore protein CENP-C reduces the diffusion constant and mobility of centromeric nucleosomes along the chromatin fiber. We subsequently interrogated how CENP-C modulates CENP-A chromatin dynamics in vivo. Overexpressing CENP-C resulted in reduced centromeric transcription and impaired loading of new CENP-A molecules. From these data, we speculate that factors altering nucleosome mobility in vitro, also correspondingly alter transcription in vivo. Subsequently, we propose a model in which variant nucleosomes encode their own diffusion kinetics and mobility, and where binding partners can suppress or enhance nucleosome mobility.

Are extraordinary nucleosome structures more ordinary than we thought?

The nucleosome is a DNA-protein assembly that is the basic unit of chromatin. A nucleosome can adopt various structures. In the canonical nucleosome structure, 145-147 bp of DNA is wrapped around a histone heterooctamer. The strong histone-DNA interactions cause the DNA to be inaccessible for nuclear processes such as transcription. Therefore, the canonical nucleosome structure has to be altered into different, non-canonical structures to increase DNA accessibility. While it is recognised that non-canonical structures do exist, these structures are not well understood. In this review, we discuss both the evidence for various non-canonical nucleosome structures in the nucleus and the factors that are believed to induce these structures. The wide range of non-canonical structures is likely to regulate the amount of accessible DNA, and thus have important nuclear functions.

Nanoscale structural organization and stoichiometry of the budding yeast kinetochore

Proper chromosome segregation is crucial for cell division. In eukaryotes, this is achieved by the kinetochore, an evolutionarily conserved multiprotein complex that physically links the DNA to spindle microtubules and takes an active role in monitoring and correcting erroneous spindle-chromosome attachments. Our mechanistic understanding of these functions and how they ensure an error-free outcome of mitosis is still limited, partly because we lack a complete understanding of the kinetochore structure in the cell. In this study, we use single-molecule localization microscopy to visualize individual kinetochore complexes in situ in budding yeast. For major kinetochore proteins, we measured their abundance and position within the metaphase kinetochore. Based on this comprehensive dataset, we propose a quantitative model of the budding yeast kinetochore. While confirming many aspects of previous reports based on bulk imaging, our results present a unifying nanoscale model of the kinetochore in budding yeast.

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.

De novo centromere formation in pericentromeric region of rice chromosome 8

Neocentromeres develop when kinetochores assemble de novo at DNA loci that are not previously associated with CenH3 nucleosomes, and can rescue rearranged chromosomes that have lost a functional centromere. The molecular mechanisms associated with neocentromere formation in plants have been elusive. Here, we developed a Xian (indica) rice line with poor growth performance in the field due to approximately 272 kb deletion that spans centromeric DNA sequences, including the centromeric satellite repeat CentO, in the centromere of chromosome 8 (Cen8). The CENH3-binding domains were expanded downstream of the original CentO position in Cen8, which revealed a de novo centromere formation in rice. The neocentromere formation avoids chromosomal regions containing functional genes. Meanwhile, canonical histone H3 was replaced by CENH3 in the regions with low CENH3 levels, and the CenH3 nucleosomes in these regions became more periodic. In addition, we identified active genes in the deleted centromeric region, which are essential for chloroplast growth and development. In summary, our results provide valuable insights into neocentromere formation and show that functional genes exist in the centromeric regions of plant chromosomes.

The roles of transcription, chromatin organisation and chromosomal processes in holocentromere establishment and maintenance

The centromere is a unique functional region on each eukaryotic chromosome where the kinetochore assembles and orchestrates microtubule attachment and chromosome segregation. Unlike monocentromeres that occupy a specific region on the chromosome, holocentromeres are diffused along the length of the chromosome. Despite being less common, holocentromeres have been verified in almost 800 nematode, insect, and plant species. Understanding of the molecular and epigenetic regulation of holocentromeres is lagging that of monocentromeres. Here we review how permissive locations for holocentromeres are determined across the genome, potentially by chromatin organisation, transcription, and non-coding RNAs, specifically in the nematode C. elegans. In addition, we discuss how holocentric CENP-A or CENP-T-containing nucleosomes are recruited and deposited, through the help of histone chaperones, licensing factors, and condensin complexes, both during de novo holocentromere establishment, and in each mitotic cell cycle. The process of resolving sister centromeres after DNA replication in holocentric organisms is also mentioned. Conservation and diversity between holocentric and monocentric organisms are highlighted, and outstanding questions are proposed.

Postmitotic G1 phase survivin drives mitogen-independent cell division of B lymphocytes

The prevailing dogma is that renewed mitogenic signaling is essential to traverse G1 phase of the cell cycle after each division. B lymphocytes undergo multiple mitotic divisions, termed clonal expansion, to expand antigen-specific cells that mediate effective immunity. Here we demonstrate that B cells that have undergone one cell division continue to proliferate even in absence of further mitogenic signals. This mitogen-independent proliferation is accompanied by an altered G1 phase marked by transcriptomic and proteomic features of G2/M. Survivin, a G2/M-specific oncogene, is required in G1 to achieve mitogen-independent proliferation.

B and T lymphocytes of the adaptive immune system undergo proliferative bursts to generate pools of antigen-specific cells for effective immunity. Here we show that in contrast to the canonical view that G1 progression signals are essential after mitosis to reenter S phase, B lymphocytes sustain several rounds of mitogen-independent cell division following the first mitosis. Such division appears to be driven by unique characteristics of the postmitotic G1 phase that has features of S and G2/M phases. Birc5 (survivin), a protein associated with chromosome segregation in G2/M, is expressed in the G1 phase of divided B cells and is necessary for mitogen-independent divisions. The partially active G1 phase and propensity for apoptosis inherited after each division may underlie rapid proliferation and cell death, which are hallmarks of B cell proliferative responses.

Quantifying cell-cycle-dependent chromatin dynamics during interphase by live 3D tracking

The study of cell cycle progression and regulation is important to our understanding of fundamental biophysics, aging, and disease mechanisms. Local chromatin movements are generally considered to be constrained and relatively consistent during all interphase stages, although recent advances in our understanding of genome organization challenge this claim. Here, we use high spatiotemporal resolution, 4D (x, y, z and time) localization microscopy by point-spread-function (PSF) engineering and deep learning-based image analysis, for live imaging of mouse embryonic fibroblast (MEF 3T3) and MEF 3T3 double Lamin A Knockout (LmnaKO) cell lines, to characterize telomere diffusion during the interphase. We detected varying constraint levels imposed on chromatin, which are prominently decreased during G0/G1. Our 4D measurements of telomere diffusion offer an effective method to investigate chromatin dynamics and reveal cell-cycle-dependent motion constraints, which may be caused by various cellular processes.

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PSF engineering allows scan-free, high spatiotemporal live 3D telomere tracking

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During the G0/G1 phase, telomere motion is less constrained than in other phases

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There is observable difference between lateral (xy) and axial (z) chromatin motion

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In Lamin A-depleted cells, motion constraint was reduced

Mechanical properties of nucleoprotein complexes determined by nanoindentation spectroscopy

The interplay between transcription factors, chromatin remodelers, 3-D organization, and mechanical properties of the chromatin fiber controls genome function in eukaryotes. Besides the canonical histones which fold the bulk of the chromatin into nucleosomes, histone variants create distinctive chromatin domains that are thought to regulate transcription, replication, DNA damage repair, and faithful chromosome segregation. Whether histone variants translate distinctive biochemical or biophysical properties to their associated chromatin structures, and whether these properties impact chromatin dynamics as the genome undergoes a multitude of transactions, is an important question in biology. Here, we describe single-molecule nanoindentation tools that we developed specifically to determine the mechanical properties of histone variant nucleosomes and their complexes. These methods join an array of cutting-edge new methods that further our quantitative understanding of the response of chromatin to intrinsic and extrinsic forces which act upon it during biological transactions in the nucleus.

RbAp46/48LIN-53 and HAT-1 are required for initial CENP-AHCP-3 deposition and de novo holocentromere formation on artificial chromosomes in Caenorhabditis elegans embryos

Foreign DNA microinjected into the Caenorhabditis elegans syncytial gonad forms episomal extra-chromosomal arrays, or artificial chromosomes (ACs), in embryos. Short, linear DNA fragments injected concatemerize into high molecular weight (HMW) DNA arrays that are visible as punctate DAPI-stained foci in oocytes, and they undergo chromatinization and centromerization in embryos. The inner centromere, inner kinetochore and spindle checkpoint components, including AIR-2, CENP-A^HCP-3, Mis18BP1^KNL-2 and BUB-1, respectively, assemble onto the nascent ACs during the first mitosis. The DNA replication efficiency of ACs improves over several cell cycles, which correlates with the improvement of kinetochore bi-orientation and proper segregation of ACs. Depletion of condensin II subunits, like CAPG-2 and SMC-4, but not the replicative helicase component, MCM-2, reduces de novo CENP-A^HCP-3 level on nascent ACs. Furthermore, H3K9ac, H4K5ac and H4K12ac are highly enriched on newly chromatinized ACs. RbAp46/48^LIN-53 and HAT-1, which affect the acetylation of histone H3 and H4, are essential for chromatinization, de novo centromere formation and segregation competency of nascent ACs. RbAp46/48^LIN-53 or HAT-1 depletion causes the loss of both CENP-A^HCP-3 and Mis18BP1^KNL-2 initial deposition at de novo centromeres on ACs. This phenomenon is different from centromere maintenance on endogenous chromosomes, where Mis18BP1^KNL-2 functions upstream of RbAp46/48^LIN-53.

Disruption of the MSL complex inhibits tumour maintenance by exacerbating chromosomal instability

Rewiring of cellular programmes in malignant cells generates cancer-specific vulnerabilities. Here, using an unbiased screening strategy aimed at identifying non-essential genes required by tumour cells to sustain unlimited proliferative capacity, we identify the male-specific lethal (MSL) acetyltransferase complex as a vulnerability of genetically unstable cancers. We find that disruption of the MSL complex and consequent loss of the associated H4K16ac mark do not substantially alter transcriptional programmes but compromise chromosome integrity and promote chromosomal instability (CIN) that progressively exhausts the proliferative potential of cancer cells through a p53-independent mechanism. This effect is dependent on pre-existing genomic instability, and normal cells are insensitive to MSL disruption. Using cell- and patient-derived xenografts from multiple cancer types, we show that excessive CIN induced by MSL disruption inhibits tumour maintenance. Our findings suggest that targeting MSL may be a valuable means to increase CIN beyond the level tolerated by cancer cells without inducing severe adverse effects in normal tissues.

FOXD1 regulates cell division in clear cell renal cell carcinoma

BACKGROUND

Forkhead transcription factors control cell growth in multiple cancer types. Foxd1 is essential for kidney development and mitochondrial metabolism, but its significance in renal cell carcinoma (ccRCC) has not been reported.

METHODS

Transcriptome data from the TCGA database was used to correlate FOXD1 expression with patient survival. FOXD1 was knocked out in the 786-O cell line and known targets were analyzed. Reduced cell growth was observed and investigated in vitro using growth rate and Seahorse XF metabolic assays and in vivo using a xenograft model. Cell cycle characteristics were determined by flow cytometry and immunoblotting. Immunostaining for TUNEL and γH2AX was used to measure DNA damage. Association of the FOXD1 pathway with cell cycle progression was investigated through correlation analysis using the TCGA database.

RESULTS

FOXD1 expression level in ccRCC correlated inversely with patient survival. Knockout of FOXD1 in 786-O cells altered expression of FOXD1 targets, particularly genes involved in metabolism (MICU1) and cell cycle progression. Investigation of metabolic state revealed significant alterations in mitochondrial metabolism and glycolysis, but no net change in energy production. In vitro growth rate assays showed a significant reduction in growth of 786-OFOXD1null. In vivo, xenografted 786-OFOXD1null showed reduced capacity for tumor formation and reduced tumor size. Cell cycle analysis showed that 786-OFOXD1null had an extended G2/M phase. Investigation of mitosis revealed a deficiency in phosphorylation of histone H3 in 786-OFOXD1null, and increased DNA damage. Genes correlate with FOXD1 in the TCGA dataset associate with several aspects of mitosis, including histone H3 phosphorylation.

CONCLUSIONS

We show that FOXD1 regulates the cell cycle in ccRCC cells by control of histone H3 phosphorylation, and that FOXD1 expression governs tumor formation and tumor growth. Transcriptome analysis supports this role for FOXD1 in ccRCC patient tumors and provides an explanation for the inverse correlation between tumor expression of FOXD1 and patient survival. Our findings reveal an important role for FOXD1 in maintaining chromatin stability and promoting cell cycle progression and provide a new tool with which to study the biology of FOXD1 in ccRCC.

Structural and dynamic mechanisms of CBF3-guided centromeric nucleosome formation

Accurate chromosome segregation relies on the specific centromeric nucleosome-kinetochore interface. In budding yeast, the centromere CBF3 complex guides the deposition of CENP-A, an H3 variant, to form the centromeric nucleosome in a DNA sequence-dependent manner. Here, we determine the structures of the centromeric nucleosome containing the native CEN3 DNA and the CBF3core bound to the canonical nucleosome containing an engineered CEN3 DNA. The centromeric nucleosome core structure contains 115 base pair DNA including a CCG motif. The CBF3core specifically recognizes the nucleosomal CCG motif through the Gal4 domain while allosterically altering the DNA conformation. Cryo-EM, modeling, and mutational studies reveal that the CBF3core forms dynamic interactions with core histones H2B and CENP-A in the CEN3 nucleosome. Our results provide insights into the structure of the budding yeast centromeric nucleosome and the mechanism of its assembly, which have implications for analogous processes of human centromeric nucleosome formation.

High-throughput AFM analysis reveals unwrapping pathways of H3 and CENP-A nucleosomes

Nucleosomes, the fundamental units of chromatin, regulate readout and expression of eukaryotic genomes. Single-molecule experiments have revealed force-induced nucleosome accessibility, but a high-resolution unwrapping landscape in the absence of external forces is currently lacking. Here, we introduce a high-throughput pipeline for the analysis of nucleosome conformations based on atomic force microscopy and automated, multi-parameter image analysis. Our data set of ∼10 000 nucleosomes reveals multiple unwrapping states corresponding to steps of 5 bp DNA. For canonical H3 nucleosomes, we observe that dissociation from one side impedes unwrapping from the other side, but in contrast to force-induced unwrapping, we find only a weak sequence-dependent asymmetry. Notably, centromeric CENP-A nucleosomes do not unwrap anti-cooperatively, in stark contrast to H3 nucleosomes. Finally, our results reconcile previous conflicting findings about the differences in height between H3 and CENP-A nucleosomes. We expect our approach to enable critical insights into epigenetic regulation of nucleosome structure and stability and to facilitate future high-throughput AFM studies that involve heterogeneous nucleoprotein complexes.

Stable inheritance of CENP-A chromatin: Inner strength versus dynamic control

Chromosome segregation during cell division is driven by mitotic spindle attachment to the centromere region on each chromosome. Centromeres form a protein scaffold defined by chromatin featuring CENP-A, a conserved histone H3 variant, in a manner largely independent of local DNA cis elements. CENP-A nucleosomes fulfill two essential criteria to epigenetically identify the centromere. They undergo self-templated duplication to reestablish centromeric chromatin following DNA replication. More importantly, CENP-A incorporated into centromeric chromatin is stably transmitted through consecutive cell division cycles. CENP-A nucleosomes have unique structural properties and binding partners that potentially explain their long lifetime in vivo. However, rather than a static building block, centromeric chromatin is dynamically regulated throughout the cell cycle, indicating that CENP-A stability is also controlled by external factors. We discuss recent insights and identify the outstanding questions on how dynamic control of the long-term stability of CENP-A ensures epigenetic centromere inheritance.

Phase-plate cryo-EM structure of the Widom 601 CENP-A nucleosome core particle reveals differential flexibility of the DNA ends

The histone H3 variant CENP-A marks centromeres epigenetically and is essential for mitotic fidelity. Previous crystallographic studies of the CENP-A nucleosome core particle (NCP) reconstituted with a human α-satellite DNA derivative revealed both DNA ends to be highly flexible, a feature important for CENP-A mitotic functions. However, recent cryo-EM studies of CENP-A NCP complexes comprising primarily Widom 601 DNA reported well-ordered DNA ends. Here, we report the cryo-EM structure of the CENP-A 601 NCP determined by Volta phase-plate imaging. The data reveal that one (‘left’) 601 DNA end is well ordered whereas the other (‘right’) end is flexible and partly detached from the histone core, suggesting sequence-dependent dynamics of the DNA termini. Indeed, a molecular dynamics simulation of the CENP-A 601 NCP confirmed the distinct dynamics of the two DNA extremities. Reprocessing the image data using two-fold symmetry yielded a cryo-EM map in which both DNA ends appeared well ordered, indicating that such an artefact may inadvertently arise if NCP asymmetry is lost during image processing. These findings enhance our understanding of the dynamic features that discriminate CENP-A from H3 nucleosomes by revealing that DNA end flexibility can be fine-tuned in a sequence-dependent manner.

Conservation of centromeric histone 3 interaction partners in plants

The loading and maintenance of centromeric histone 3 (CENH3) at the centromere are critical processes ensuring appropriate kinetochore establishment and equivalent segregation of the homologous chromosomes during cell division. CENH3 loss of function is lethal, whereas mutations in the histone fold domain are tolerated and lead to chromosome instability and chromosome elimination in embryos derived from crosses with wild-type pollen. A wide range of proteins in yeast and animals have been reported to interact with CENH3. The histone fold domain-interacting proteins are potentially alternative targets for the engineering of haploid inducer lines, which may be important when CENH3 mutations are not well supported by a given crop. Here, we provide an overview of the corresponding plant orthologs or functional homologs of CENH3-interacting proteins. We also list putative CENH3 post-translational modifications that are also candidate targets for modulating chromosome stability and inheritance.

Epigenetic regulation of centromere function

The centromere is a specialized region on the chromosome that directs equal chromosome segregation. Centromeres are usually not defined by DNA sequences alone. How centromere formation and function are determined by epigenetics is still not fully understood. Active centromeres are often marked by the presence of centromeric-specific histone H3 variant, centromere protein A (CENP-A). How CENP-A is assembled into the centromeric chromatin during the cell cycle and propagated to the next cell cycle or the next generation to maintain the centromere function has been intensively investigated. In this review, we summarize current understanding of how post-translational modifications of CENP-A and other centromere proteins, centromeric and pericentric histone modifications, non-coding transcription and transcripts contribute to centromere function, and discuss their intricate relationships and potential feedback mechanisms.

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.

Synergistic Coordination of Chromatin Torsional Mechanics and Topoisomerase Activity

DNA replication in eukaryotes generates DNA supercoiling, which may intertwine (braid) daughter chromatin fibers to form precatenanes, posing topological challenges during chromosome segregation. The mechanisms that limit precatenane formation remain unclear. By making direct torque measurements, we demonstrate that the intrinsic mechanical properties of chromatin play a fundamental role in dictating precatenane formation and regulating chromatin topology. Whereas a single chromatin fiber is torsionally soft, a braided fiber is torsionally stiff, indicating that supercoiling on chromatin substrates is preferentially directed in front of the fork during replication. We further show that topoisomerase II relaxation displays a strong preference for a single chromatin fiber over a braided fiber. These results suggest a synergistic coordination-the mechanical properties of chromatin inherently suppress precatenane formation during replication elongation by driving DNA supercoiling ahead of the fork, where supercoiling is more efficiently removed by topoisomerase II. VIDEO ABSTRACT.

Phosphorylation of Drosophila CENP-A on serine 20 regulates protein turn-over and centromere-specific loading

Centromeres are specialized chromosomal regions epigenetically defined by the presence of the histone H3 variant CENP-A. CENP-A is required for kinetochore formation which is essential for chromosome segregation during mitosis. Spatial restriction of CENP-A to the centromere is tightly controlled. Its overexpression results in ectopic incorporation and the formation of potentially deleterious neocentromeres in yeast, flies and in various human cancers. While the contribution of posttranslational modifications of CENP-A to these processes has been studied in yeast and mammals to some extent, very little is known about Drosophila melanogaster. Here, we show that CENP-A is phosphorylated at serine 20 (S20) by casein kinase II and that in mitotic cells, the phosphorylated form is enriched on chromatin. Importantly, our results reveal that S20 phosphorylation regulates the turn-over of prenucleosomal CENP-A by the SCFPpa-proteasome pathway and that phosphorylation promotes removal of CENP-A from ectopic but not from centromeric sites in chromatin. We provide multiple lines of evidence for a crucial role of S20 phosphorylation in controlling restricted incorporation of CENP-A into centromeric chromatin in flies. Modulation of the phosphorylation state of S20 may provide the cells with a means to fine-tune CENP-A levels in order to prevent deleterious loading to extra-centromeric sites.

CENP-A nucleosome clusters form rosette-like structures around HJURP during G1

CENP-A is an essential histone H3 variant that epigenetically marks the centromeric region of chromosomes. Here we show that CENP-A nucleosomes form characteristic clusters during the G1 phase of the cell cycle. 2D and 3D super-resolution microscopy and segmentation analysis reveal that these clusters encompass a globular rosette-like structure, which evolves into a more compact structure in late G1. The rosette-like clusters contain numerous CENP-A molecules and form a large cellular structure of ∼250-300 nm diameter with remarkably similar shapes for each centromere. Co-localization analysis shows that HJURP, the CENP-A chaperone, is located in the center of the rosette and serves as a nucleation point. The discovery of an HJURP-mediated CENP-A nucleation in human cells and its structural description provide important insights into the mechanism of CENP-A deposition and the organization of CENP-A chromatin in the centromeric region.

The regulation of chromosome segregation via centromere loops

Biophysical studies of the yeast centromere have shown that the organization of the centromeric chromatin plays a crucial role in maintaining proper tension between sister kinetochores during mitosis. While centromeric chromatin has traditionally been considered a simple spring, recent work reveals the centromere as a multifaceted, tunable shock absorber. Centromeres can differ from other regions of the genome in their heterochromatin state, supercoiling state, and enrichment of structural maintenance of chromosomes (SMC) protein complexes. Each of these differences can be utilized to alter the effective stiffness of centromeric chromatin. In budding yeast, the SMC protein complexes condensin and cohesin stiffen chromatin by forming and cross-linking chromatin loops, respectively, into a fibrous structure resembling a bottlebrush. The high density of the loops compacts chromatin while spatially isolating the tension from spindle pulling forces to a subset of the chromatin. Paradoxically, the molecular crowding of chromatin via cohesin and condensin also causes an outward/poleward force. The structure allows the centromere to act as a shock absorber that buffers the variable forces generated by dynamic spindle microtubules. Based on the distribution of SMCs from bacteria to human and the conserved distance between sister kinetochores in a wide variety of organisms (0.4 to 1 micron), we propose that the bottlebrush mechanism is the foundational principle for centromere function in eukaryotes.

DNA replication acts as an error correction mechanism to maintain centromere identity by restricting CENP-A to centromeres

Chromatin assembled with the histone H3 variant CENP-A is the heritable epigenetic determinant of human centromere identity. Using genome-wide mapping and reference models for 23 human centromeres, CENP-A binding sites are identified within the megabase-long, repetitive α-satellite DNAs at each centromere. CENP-A is shown in early G1 to be assembled into nucleosomes within each centromere and onto 11,390 transcriptionally active sites on the chromosome arms. DNA replication is demonstrated to remove ectopically loaded, non-centromeric CENP-A. In contrast, tethering of centromeric CENP-A to the sites of DNA replication through the constitutive centromere associated network (CCAN) is shown to enable precise reloading of centromere-bound CENP-A onto the same DNA sequences as in its initial prereplication loading. Thus, DNA replication acts as an error correction mechanism for maintaining centromere identity through its removal of non-centromeric CENP-A coupled with CCAN-mediated retention and precise reloading of centromeric CENP-A.

Seeing is believing: our evolving view of kinetochore structure, composition, and assembly

This review highlights three recent trends in the field of kinetochore biology: the proliferation of structural data for kinetochore protein complexes (including CBF3, Dam1c, Mis12c^MIND, and CENP-NL^Chl4/Iml3); the growing consensus that the kinetochore is a dynamic structure whose composition changes as the cell cycle progresses; and the mounting evidence of multiple pathways whereby the microtubule-binding elements of the outer kinetochore may be recruited by inner kinetochore proteins. Our focus is on the two best-studied systems in the field: human and budding yeast kinetochores. This review will demonstrate the remarkable similarity of these two systems, as well as their intriguing differences.

The Oligomerization Landscape of Histones

In eukaryotes, DNA is packaged within nucleosomes. The DNA of each nucleosome is typically centered around an octameric histone protein core: one central tetramer plus two separate dimers. Studying the assembly mechanisms of histones is essential for understanding the dynamics of entire nucleosomes and higher-order DNA packaging. Here, we investigate canonical histone assembly and that of the centromere-specific histone variant, centromere protein A (CENP-A), using molecular dynamics simulations. We quantitatively characterize their thermodynamical and dynamical features, showing that two H3/H4 dimers form a structurally floppy, weakly bound complex, the latter exhibiting large instability around the central interface manifested via a swiveling motion of two halves. This finding is consistent with the recently observed DNA handedness flipping of the tetrasome. In contrast, the variant CENP-A encodes distinctive stability to its tetramer with a rigid but twisted interface compared to the crystal structure, implying diverse structural possibilities of the histone variant. Interestingly, the observed tetramer dynamics alter significantly and appear to reach a new balance when H2A/H2B dimers are present. Furthermore, we found that the preferred structure for the (CENP-A/H4)₂ tetramer is incongruent with the octameric structure, explaining many of the unusual dynamical behaviors of the CENP-A nucleosome. In all, these data reveal key mechanistic insights and structural details for the assembly of canonical and variant histone tetramers and octamers, providing theoretical quantifications and physical interpretations for longstanding and recent experimental observations. Based on these findings, we propose different chaperone-assisted binding and nucleosome assembly mechanisms for the canonical and CENP-A histone oligomers.

AFM study: Cell cycle and probe geometry influences nanomechanical characterization of Panc1 cells

Atomic force microscope (AFM) is emerging as an immensely promising tool to study the cellular morphology with a nanometer scale resolution and to analyze nanomechanical properties (NPs) at various physiological conditions. Advancement of AFM technology enables studying living cells and differentiating cancer cell from normal cells based on topography and NPs. Though the trend overlaps from different literature; numerical values of nanomechanical readouts depict variations over a wide range. These anomalies are associated with the experimental setup under study. In this manuscript, we have identified heterogeneity in cell culture system in addition to the selection of AFM probe with specific tip geometry as the major contributors to the above mentioned anomalies. To test our hypothesis, we have used Panc1 cells, which is a pancreatic ductal adenocarcinoma cell type. Our results suggest that the cellular morphology, membrane roughness and NPs calculated from AFM study are distinctly influenced by cell cycle. Furthermore, we found that the NPs readout is also significantly associated with AFM tip geometries. The cells were found to be softer in their early resting phase when indented with pyramidal probe and became increasingly stiffer as they progressed through the cell cycles. On the contrary, when indented with the spherical probe, cells in G0/G1 phase were observed to be the stiffest. Such an exhaustive study of the role of cell cycle in influencing the NPs in Panc1 cell line along with the impact of tip geometry on NPs is the first of its kind, to the best of our knowledge.

Repetitive Fragile Sites: Centromere Satellite DNA As a Source of Genome Instability in Human Diseases

Maintenance of an intact genome is essential for cellular and organismal homeostasis. The centromere is a specialized chromosomal locus required for faithful genome inheritance at each round of cell division. Human centromeres are composed of large tandem arrays of repetitive alpha-satellite DNA, which are often sites of aberrant rearrangements that may lead to chromosome fusions and genetic abnormalities. While the centromere has an essential role in chromosome segregation during mitosis, the long and repetitive nature of the highly identical repeats has greatly hindered in-depth genetic studies, and complete annotation of all human centromeres is still lacking. Here, we review our current understanding of human centromere genetics and epigenetics as well as recent investigations into the role of centromere DNA in disease, with a special focus on cancer, aging, and human immunodeficiency⁻centromeric instability⁻facial anomalies (ICF) syndrome. We also highlight the causes and consequences of genomic instability at these large repetitive arrays and describe the possible sources of centromere fragility. The novel connection between alpha-satellite DNA instability and human pathological conditions emphasizes the importance of obtaining a truly complete human genome assembly and accelerating our understanding of centromere repeats' role in physiology and beyond.

Inheritance of CENP-A Nucleosomes during DNA Replication Requires HJURP

Centromeric chromatin defines the site of kinetochore formation and ensures faithful chromosome segregation. Centromeric identity is epigenetically specified by the incorporation of CENP-A nucleosomes. DNA replication presents a challenge for inheritance of centromeric identity because nucleosomes are removed to allow for replication fork progression. Despite this challenge, CENP-A nucleosomes are stably retained through S phase. We used BioID to identify proteins transiently associated with CENP-A during DNA replication. We found that during S phase, HJURP transiently associates with centromeres and binds to pre-existing CENP-A, suggesting a distinct role for HJURP in CENP-A retention. We demonstrate that HJURP is required for centromeric nucleosome inheritance during S phase. HJURP co-purifies with the MCM2-7 helicase complex and, together with the MCM2 subunit, binds CENP-A simultaneously. Therefore, pre-existing CENP-A nucleosomes require an S phase function of the HJURP chaperone and interaction with MCM2 to ensure faithful inheritance of centromere identity through DNA replication.

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.

Insights into Centromere DNA Bending Revealed by the Cryo-EM Structure of the Core Centromere Binding Factor 3 with Ndc10

The centromere binding factor 3 (CBF3) complex binds the third centromere DNA element in organisms with point centromeres, such as S. cerevisiae. It is an essential complex for assembly of the kinetochore in these organisms, as it facilitates genetic centromere specification and allows association of all other kinetochore components. We determined high-resolution structures of the core complex of CBF3 alone and in association with a monomeric construct of Ndc10, using cryoelectron microscopy (cryo-EM). We identify the DNA-binding site of the complex and present a model in which CBF3 induces a tight bend in centromeric DNA, thus facilitating assembly of the centromeric nucleosome.

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Cryo-EM studies of CBF3 reveal the core complex has a deep asymmetric channel

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The size, conservation, and charge of the channel suggest that it binds centromere DNA

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Unique insertions in the Ctf13 F box provide the binding site for one Ndc10 monomer

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The Ndc10 DNA-binding site is in plane with and perpendicular to the CBF3 channel

Multimodal Light Microscopy Approaches to Reveal Structural and Functional Properties of Promyelocytic Leukemia Nuclear Bodies

The promyelocytic leukemia (pml) gene product PML is a tumor suppressor localized mainly in the nucleus of mammalian cells. In the cell nucleus, PML seeds the formation of macromolecular multiprotein complexes, known as PML nuclear bodies (PML NBs). While PML NBs have been implicated in many cellular functions including cell cycle regulation, survival and apoptosis their role as signaling hubs along major genome maintenance pathways emerged more clearly. However, despite extensive research over the past decades, the precise biochemical function of PML in these pathways is still elusive. It remains a big challenge to unify all the different previously suggested cellular functions of PML NBs into one mechanistic model. With the advent of genetically encoded fluorescent proteins it became possible to trace protein function in living specimens. In parallel, a variety of fluorescence fluctuation microscopy (FFM) approaches have been developed which allow precise determination of the biophysical and interaction properties of cellular factors at the single molecule level in living cells. In this report, we summarize the current knowledge on PML nuclear bodies and describe several fluorescence imaging, manipulation, FFM, and super-resolution techniques suitable to analyze PML body assembly and function. These include fluorescence redistribution after photobleaching, fluorescence resonance energy transfer, fluorescence correlation spectroscopy, raster image correlation spectroscopy, ultraviolet laser microbeam-induced DNA damage, erythrocyte-mediated force application, and super-resolution microscopy approaches. Since most if not all of the microscopic equipment to perform these techniques may be available in an institutional or nearby facility, we hope to encourage more researches to exploit sophisticated imaging tools for their research in cancer biology.

Posttranslational mechanisms controlling centromere function and assembly

Accurate chromosome segregation is critical to ensure the faithful inheritance of the genome during cell division. Human chromosomes distinguish the location of the centromere from general chromatin by the selective assembly of CENP-A containing nucleosomes at the active centromere. The location of centromeres in most higher eukaryotes is determined epigenetically, independent of DNA sequence. CENP-A containing centromeric chromatin provides the foundation for assembly of the kinetochore that mediates chromosome attachment to the microtubule spindle and controls cell cycle progression in mitosis. Here we review recent work demonstrating the role of posttranslational modifications on centromere function and CENP-A inheritance via the direct modification of the CENP-A nucleosome and pre-nucleosomal complexes, the modification of the CENP-A deposition machinery and the modification of histones within existing centromeres.

CENP-C/H/I/K/M/T/W/N/L and hMis12 but not CENP-S/X participate in complex formation in the nucleoplasm of living human interphase cells outside centromeres

Kinetochore proteins assemble onto centromeric chromatin and regulate DNA segregation during cell division. The inner kinetochore proteins bind centromeres while most outer kinetochore proteins assemble at centromeres during mitosis, connecting the complex to microtubules. Here, we measured the co-migration between protein pairs of the constitutive centromere associated network (CCAN) and hMis12 complexes by fluorescence cross-correlation spectroscopy (FCCS) in the nucleoplasm outside centromeres in living human interphase cells. FCCS is a method that can tell if in living cells two differently fluorescently labelled molecules migrate independently, or co-migrate and thus are part of one and the same soluble complex. We also determined the apparent dissociation constants (Kd) of the hetero-dimers CENP-T/W and CENP-S/X. We measured co-migration between CENP-K and CENP-T as well as between CENP-M and CENP-T but not between CENP-T/W and CENP-S/X. Furthermore, CENP-C co-migrated with CENP-H, and CENP-K with CENP-N as well as with CENP-L. Thus, in the nucleoplasm outside centromeres, a large fraction of the CENP-H/I/K/M proteins interact with CENP-C, CENP-N/L and CENP-T/W but not with CENP-S/X. Our FCCS analysis of the Mis12 complex showed that hMis12, Nsl1, Dsn1 and Nnf1 also form a complex outside centromeres of which at least hMis12 associated with the CENP-C/H/I/K/M/T/W/N/L complex.

The Art of War: harnessing the epigenome against cancer

Histone chaperones are indispensable regulators of chromatin structure and function. Recent work has shown that they are frequently mis-regulated in cancer, which can have profound consequences on tumor growth and survival. Here, we focus on chaperones for the essential H3 histone variants H3.3 and CENP-A, specifically HIRA, DAXX/ATRX, DEK, and HJURP. This review summarizes recent studies elucidating their roles in regulating chromatin and discusses how cancer-specific chromatin interactions can be exploited to target cancer cells.

Remarkable Evolutionary Plasticity of Centromeric Chromatin

Centromeres were familiar to cell biologists in the late 19th century, but for most eukaryotes the basis for centromere specification has remained enigmatic. Much attention has been focused on the cenH3 (CENP-A) histone variant, which forms the foundation of the centromere. To investigate the DNA sequence requirements for centromere specification, we applied a variety of epigenomic approaches, which have revealed surprising diversity in centromeric chromatin properties. Whereas each point centromere of budding yeast is occupied by a single precisely positioned tetrameric nucleosome with one cenH3 molecule, the "regional" centromeres of fission yeast contain unphased presumably octameric nucleosomes with two cenH3s. In Caenorhabditis elegans, kinetochores assemble all along the chromosome at sites of cenH3 nucleosomes that resemble budding yeast point centromeres, whereas holocentric insects lack cenH3 entirely. The "satellite" centromeres of most animals and plants consist of cenH3-containing particles that are precisely positioned over homogeneous tandem repeats, but in humans, different α-satellite subfamilies are occupied by CENP-A nucleosomes with very different conformations. We suggest that this extraordinary evolutionary diversity of centromeric chromatin architectures can be understood in terms of the simplicity of the task of equal chromosome segregation that is continually subverted by selfish DNA sequences.

SGT1-HSP90 complex is required for CENP-A deposition at centromeres

The centromere plays an essential role in accurate chromosome segregation, and defects in its function lead to aneuploidy and thus cancer. The centromere-specific histone H3 variant CENP-A is proposed to be the epigenetic mark of the centromere, as active centromeres require CENP-A–containing nucleosomes to direct the recruitment of multiple kinetochore proteins. CENP-A K124 ubiquitylation, mediated by CUL4A-RBX1-COPS8 E3 ligase activity, is required for CENP-A deposition at the centromere. However, the mechanism that controls the E3 ligase activity of the CUL4A-RBX1-COPS8 complex remains obscure. We have discovered that the SGT1-HSP90 complex is required for recognition of CENP-A by COPS8. Thus, the SGT1-HSP90 complex contributes to the E3 ligase activity of the CUL4A complex that is necessary for CENP-A ubiquitylation and CENP-A deposition at the centromere.

CENP-A regulates chromosome segregation during the first meiosis of mouse oocytes

Proper chromosome separation in both mitosis and meiosis depends on the correct connection between kinetochores of chromosomes and spindle microtubules. Kinetochore dysfunction can lead to unequal distribution of chromosomes during cell division and result in aneuploidy, thus kinetochores are critical for faithful segregation of chromosomes. Centromere protein A (CENP-A) is an important component of the inner kinetochore plate. Multiple studies in mitosis have found that deficiencies in CENP-A could result in structural and functional changes of kinetochores, leading to abnormal chromosome segregation, aneuploidy and apoptosis in cells. Here we report the expression and function of CENP-A during mouse oocyte meiosis. Our study found that microinjection of CENP-A blocking antibody resulted in errors of homologous chromosome segregation and caused aneuploidy in eggs. Thus, our findings provide evidence that CENP-A is critical for the faithful chromosome segregation during mammalian oocyte meiosis.

Chromatin assembly: Journey to the CENter of the chromosome

All eukaryotic genomes are packaged into basic units of DNA wrapped around histone proteins called nucleosomes. The ability of histones to specify a variety of epigenetic states at defined chromatin domains is essential for cell survival. The most distinctive type of chromatin is found at centromeres, which are marked by the centromere-specific histone H3 variant CENP-A. Many of the factors that regulate CENP-A chromatin have been identified; however, our understanding of the mechanisms of centromeric nucleosome assembly, maintenance, and reorganization remains limited. This review discusses recent insights into these processes and draws parallels between centromeric and noncentromeric chromatin assembly mechanisms.

Internal modifications in the CENP-A nucleosome modulate centromeric dynamics

Background

Posttranslational modifications of core histones are correlated with changes in transcriptional status, chromatin fiber folding, and nucleosome dynamics. However, within the centromere-specific histone H3 variant CENP-A, few modifications have been reported, and their functions remain largely unexplored. In this multidisciplinary report, we utilize in silico computational and in vivo approaches to dissect lysine 124 of human CENP-A, which was previously reported to be acetylated in advance of replication.

Results

Computational modeling demonstrates that acetylation of K124 causes tightening of the histone core and hinders accessibility to its C-terminus, which in turn diminishes CENP-C binding. Additionally, CENP-A K124ac/H4 K79ac containing nucleosomes are prone to DNA sliding. In vivo experiments using a CENP-A acetyl or unacetylatable mimic (K124Q and K124A, respectively) reveal alterations in CENP-C levels and a modest increase in mitotic errors. Furthermore, mutation of K124 results in alterations in centromeric replication timing. Purification of native CENP-A proteins followed by mass spectrometry analysis reveals that while CENP-A K124 is acetylated at G1/S, it switches to monomethylation during early S and mid-S phases. Finally, we provide evidence implicating the histone acetyltransferase (HAT) p300 in this cycle.

Conclusions

Taken together, our data suggest that cyclical modifications within the CENP-A nucleosome contribute to the binding of key kinetochore proteins, the integrity of mitosis, and centromeric replication. These data support the paradigm that modifications in histone variants can influence key biological processes.

Electronic supplementary material

The online version of this article (doi:10.1186/s13072-017-0124-6) contains supplementary material, which is available to authorized users.

Human centromeric CENP-A chromatin is a homotypic, octameric nucleosome at all cell cycle points

In this study, the authors use new reference models for 23 human centromeres and find that at all cell cycle phases centromeric CENP-A chromatin complexes are octameric nucleosomes with two molecules of CENP-A. This finding refutes previous models that have suggested that hemisomes may briefly transition to octameric nucleosomes.

Chromatin assembled with centromere protein A (CENP-A) is the epigenetic mark of centromere identity. Using new reference models, we now identify sites of CENP-A and histone H3.1 binding within the megabase, α-satellite repeat–containing centromeres of 23 human chromosomes. The overwhelming majority (97%) of α-satellite DNA is found to be assembled with histone H3.1–containing nucleosomes with wrapped DNA termini. In both G1 and G2 cell cycle phases, the 2–4% of α-satellite assembled with CENP-A protects DNA lengths centered on 133 bp, consistent with octameric nucleosomes with DNA unwrapping at entry and exit. CENP-A chromatin is shown to contain equimolar amounts of CENP-A and histones H2A, H2B, and H4, with no H3. Solid-state nanopore analyses show it to be nucleosomal in size. Thus, in contrast to models for hemisomes that briefly transition to octameric nucleosomes at specific cell cycle points or heterotypic nucleosomes containing both CENP-A and histone H3, human CENP-A chromatin complexes are octameric nucleosomes with two molecules of CENP-A at all cell cycle phases.

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.

Chromatin dynamics during the cell cycle at centromeres

Centromeric chromatin undergoes major changes in composition and architecture during each cell cycle. These changes in specialized chromatin facilitate kinetochore formation in mitosis to ensure proper chromosome segregation. Thus, proper orchestration of centromeric chromatin dynamics during interphase, including replication in S phase, is crucial. We provide the current view concerning the centromeric architecture associated with satellite repeat sequences in mammals and its dynamics during the cell cycle. We summarize the contributions of deposited histone variants and their chaperones, other centromeric components - including proteins and their post-translational modifications, and RNAs - and we link the expression and deposition timing of each component during the cell cycle. Because neocentromeres occur at ectopic sites, we highlight how cell cycle processes can go wrong, leading to neocentromere formation and potentially disease.

CENP-A Modifications on Ser68 and Lys124 Are Dispensable for Establishment, Maintenance, and Long-Term Function of Human Centromeres

CENP-A is a histone H3 variant key to epigenetic specification of mammalian centromeres. Using transient overexpression of CENP-A mutants, two recent reports in Developmental Cell proposed essential centromere functions for post-translational modifications of human CENP-A. Phosphorylation at Ser68 was proposed to have an essential role in CENP-A deposition at centromeres. Blockage of ubiquitination at Lys124 was proposed to abrogate localization of CENP-A to the centromere. Following gene inactivation and replacement in human cells, we demonstrate that CENP-A mutants that cannot be phosphorylated at Ser68 or ubiquitinated at Lys124 assemble efficiently at centromeres during G1, mediate early events in centromere establishment at an ectopic chromosomal locus, and maintain centromere function indefinitely. Thus, neither Ser68 nor Lys124 post-translational modification is essential for long-term centromere identity, propagation, cell-cycle-dependent deposition, maintenance, function, or mediation of early steps in centromere establishment.

Post-translational Modifications of Centromeric Chromatin

Regulation of chromatin structures is important for the control of DNA processes such as gene expression, and misregulation of chromatin is implicated in diverse diseases. Covalent post-translational modifications of histones are a prominent way to regulate chromatin structure and different chromatin regions bear their specific signature of histone modifications. The composition of centromeric chromatin is significantly different from other chromatin structures and mainly defined by the presence of the histone H3-variant CENP-A. Here we summarize the composition of centromeric chromatin and what we know about its differential regulation by post-translational modifications.

Mass Spectrometry-Based Methodology for Identification of Native Histone Variant Modifications From Mammalian Tissues and Solid Tumors

Histone posttranslational modifications (PTMs) are key epigenetic marks involved in gene silencing or activation. Histone modifications impact chromatin organization and transcriptional processes through the changes in charge density between histones and DNA. They also serve as recognition and binding sites for specific binding proteins. Histone tails and globular cores contain many basic amino acid residues, which are subject to various dynamic modifications, making the modification repertoire extremely diverse. Consequently, determination of histone PTM identity and quantity has been a challenging task. In recent years, mass spectrometry-based methods have proven useful in histone PTM characterization. This chapter provides a brief overview of these methods and describes the approach to analyze the PTMs of the histone variant CENP-A, essential for the cell cycle progression, when present in minute amounts from tumor and mammalian tissues. Because this method does not rely on antibody-based immunopurification, we anticipate that these tools could be readily adaptable to the investigation to other histone variants in a range of mammalian tissues and solid tumors.