Peter E. Warburton

Immunolocalization of CENP-A suggests a distinct nucleosome structure at the inner kinetochore plate of active centromeres

The trilaminar kinetochore directs the segregation of chromosomes in mitosis and meiosis. Despite its importance, the molecular architecture of this structure remains poorly understood [1]. The best known component of the kinetochore plates is CENP-C, a protein that is required for kinetochore assembly [2], but whose molecular role in kinetochore structure and function is unknown. Here we have raised for the first time monospecific antisera to CENP-A [3], a 17 kD centromere-specific histone variant that is 62% identical to the carboxy-terminal domain of histone H3 [4,5] and that resembles the yeast centromeric component CSE4 [6]. We have found by simultaneous immunofluorescence with centromere antigens of known ultrastructural location that CENP-A is concentrated in the region of the inner kinetochore plate at active centromeres. Because CENP-A was previously shown to co-purify with nucleosomes [7], our data suggest a specific nucleosomal substructure for the kinetochore. In human cells, these kinetochore-specific nucleosomes are enriched in alpha-satellite DNA [8]. However, the association of CENP-A with neocentromeres lacking detectable alpha-satellite DNA, and the lack of CENP-A association with alpha-satellite-rich inactive centromeres of dicentric chromosomes together suggest that CENP-A association with kinetochores is unlikely to be determined solely by DNA sequence recognition. We speculate that CENP-A binding could be a consequence of epigenetic tagging of mammalian centromeres.

The mouse X chromosome is enriched for multicopy testis genes showing postmeiotic expression

According to the prevailing view, mammalian X chromosomes are enriched in spermatogenesis genes expressed before meiosis and deficient in spermatogenesis genes expressed after meiosis. The paucity of postmeiotic genes on the X chromosome has been interpreted as a consequence of meiotic sex chromosome inactivation (MSCI)—the complete silencing of genes on the XY bivalent at meiotic prophase. Recent studies have concluded that MSCI-initiated silencing persists beyond meiosis and that most genes on the X chromosome remain repressed in round spermatids. Here, we report that 33 multicopy gene families, representing ∼273 mouse X-linked genes, are expressed in the testis and that this expression is predominantly in postmeiotic cells. RNA FISH and microarray analysis show that the maintenance of X chromosome postmeiotic repression is incomplete. Furthermore, X-linked multicopy genes exhibit a similar degree of expression as autosomal genes. Thus, not only is the mouse X chromosome enriched for spermatogenesis genes functioning before meiosis, but in addition, ∼18% of mouse X-linked genes are expressed in postmeiotic cells.

Chromosomal dynamics of human neocentromere formation

Neocentromeres are rare human chromosomal aberrations where a new centromere has formed in a previously non-centromeric location. The emergence of new centromeres on a chromosome that already contains an endogenous centromere would be a highly deleterious event which would lead to dicentricity and mitotic instability. Nonetheless, neocentromere formation appears to provide a mechanism for the acquisition of a new centromere. Neocentromeres are most often observed on chromosomal arm fragments that have separated from an endogenous centromere, and therefore actually lead to mitotic stability of what would have been an acentric fragment. Neocentromeres have recently also been observed on apparently unrearranged chromosomes where the endogenous centromere has been inactivated. Furthermore, the process of centromere repositioning during primate chromosomal evolution may depend on the acquisition and subsequent fixation of neocentromeres. This remarkable plasticity in the position of centromeres has important implications for human cytogenetics and chromosome evolution, and provides an opportunity to further our understanding of the process of centromere formation and structure.

The octamer is the major form of CENP-A nucleosomes at human centromeres.

The centromere is the chromosomal locus that ensures fidelity in genome transmission at cell division. Centromere protein A (CENP-A) is a histone H3 variant that specifies centromere location independently of DNA sequence. Conflicting evidence has emerged regarding the histone composition and stoichiometry of CENP-A nucleosomes. Here we show that the predominant form of the CENP-A particle at human centromeres is an octameric nucleosome. CENP-A nucleosomes are very highly phased on α-satellite 171-base-pair monomers at normal centromeres and also display strong positioning at neocentromeres. At either type of functional centromere, CENP-A nucleosomes exhibit similar DNA-wrapping behavior, as do octameric CENP-A nucleosomes reconstituted with recombinant components, having looser DNA termini than those on conventional nucleosomes containing canonical histone H3. Thus, the fundamental unit of the chromatin that epigenetically specifies centromere location in mammals is an octameric nucleosome with loose termini.

Molecular cytogenetic analysis of eight inversion duplications of human chromosome 13q that each contain a neocentromere.

Neocentromeres are fully functional centromeres that have arisen in previously noncentromeric chromosomal locations on rearranged chromosomes. The formation of neocentromeres results in the mitotic stability of chromosomal fragments that do not contain endogenous centromeres and that would normally be lost. Here we describe a unique collection of eight independent patient-derived cell lines, each of which contains a neocentromere on a supernumerary inversion duplication of a portion of human chromosome 13q. Findings in these patients reveal insight into the clinical manifestations associated with polysomy for portions of chromosome 13q. The results of FISH and immunofluorescent analysis of the neocentromeres in these chromosomes confirm the lack of alpha-satellite DNA and the presence of CENtromere proteins (CENP)-C, -E, and hMAD2. The positions of the inversion breakpoints in these chromosomes have been placed onto the physical map of chromosome 13, by means of FISH mapping with cosmid probes. These cell lines define, within chromosome 13q, at least three distinct locations where neocentromeres have formed, with five independent neocentromeres in band 13q32, two in band 13q21, and one in band 13q31. The results of examination of the set of 40 neocentromere-containing chromosomes that have thus far been described, including the 8 neocentromere-containing chromosomes from chromosome 13q that are described in the present study, suggest that chromosome 13q has an increased propensity for neocentromere formation, relative to some other human chromosomes. These neocentromeres will provide the means for testing hypotheses about sequence requirements for human centromere formation.

Transmission of a fully functional human neocentromere through three generations.

An unusual Y chromosome with a primary constriction inside the long-arm heterochromatin was found in the amniocytes of a 38-year-old woman. The same Y chromosome was found in her husband and brother-in-law, thus proving that it was already present in the father. FISH with alphoid DNA showed hybridization signals at the usual position of the Y centromere but not at the primary constriction. Centromere proteins (CENP)-A, CENP-C, and CENP-E could not be detected at the site of the canonic centromere but were present at the new constriction, whereas CENP-B was not detected on this Y chromosome. Experiments with 82 Y-specific loci distributed throughout the chromosome confirmed that no gross deletion or rearrangement had taken place, and that the Y chromosome belonged to a haplogroup whose members have a mean alphoid array of 770 kb (range 430-1,600 kb), whereas that of this case was approximately 250 kb. Thus, this Y chromosome appeared to be deleted for part of the alphoid DNA. It seems likely that this deletion was responsible for the silencing of the normal centromere and that the activation of the neocentromere prevented the loss of this chromosome. Alternatively, neocentromere activation could have occurred first and stimulated inactivation of the normal centromere by partial deletion. Whatever the mechanism, the presence of this chromosome in three generations demonstrates that it functions sufficiently well in mitosis for male sex determination and fertility and that neocentromeres can be transmitted normally at meiosis.

Co-localization of CENP-C and CENP-H to discontinuous domains of CENP-A chromatin at human neocentromeres

BackgroundMammalian centromere formation is dependent on chromatin that contains centromere protein (CENP)-A, which is the centromere-specific histone H3 variant. Human neocentromeres have acquired CENP-A chromatin epigenetically in ectopic chromosomal locations on low-copy complex DNA. Neocentromeres permit detailed investigation of centromeric chromatin organization that is not possible in the highly repetitive alpha satellite DNA present at endogenous centromeres.ResultsWe have examined the distribution of CENP-A, as well as two additional centromeric chromatin-associated proteins (CENP-C and CENP-H), across neocentromeric DNA using chromatin immunoprecipitation (ChIP) on CHIP assays on custom genomic microarrays at three different resolutions. Analysis of two neocentromeres using a contiguous bacterial artificial chromosome (BAC) microarray spanning bands 13q31.3 to 13q33.1 shows that both CENP-C and CENP-H co-localize to the CENP-A chromatin domain. Using a higher resolution polymerase chain reaction (PCR)-amplicon microarray spanning the neocentromere, we find that the CENP-A chromatin is discontinuous, consisting of a major domain of about 87.8 kilobases (kb) and a minor domain of about 13.2 kb, separated by an approximately 158 kb region devoid of CENPs. Both CENP-A domains exhibit co-localization of CENP-C and CENP-H, defining a distinct inner kinetochore chromatin structure that is consistent with higher order chromatin looping models at centromeres. The PCR microarray data suggested varying density of CENP-A nucleosomes across the major domain, which was confirmed using a higher resolution oligo-based microarray.ConclusionCentromeric chromatin consists of several CENP-A subdomains with highly discontinuous CENP-A chromatin at both the level of individual nucleosomes and at higher order chromatin levels, raising questions regarding the overall structure of centromeric chromatin.

Independent centromere formation in a capricious, gene-free domain of chromosome 13q21 in Old World monkeys and pigs

BackgroundEvolutionary centromere repositioning and human analphoid neocentromeres occurring in clinical cases are, very likely, two stages of the same phenomenon whose properties still remain substantially obscure. Chromosome 13 is the chromosome with the highest number of neocentromeres. We reconstructed the mammalian evolutionary history of this chromosome and characterized two human neocentromeres at 13q21, in search of information that could improve our understanding of the relationship between evolutionarily new centromeres, inactivated centromeres, and clinical neocentromeres.ResultsChromosome 13 evolution was studied, using FISH experiments, across several diverse superordinal phylogenetic clades spanning >100 million years of evolution. The analysis revealed exceptional conservation among primates (hominoids, Old World monkeys, and New World monkeys), Carnivora (cat), Perissodactyla (horse), and Cetartiodactyla (pig). In contrast, the centromeres in both Old World monkeys and pig have apparently repositioned independently to a central location (13q21). We compared these results to the positions of two human 13q21 neocentromeres using chromatin immunoprecipitation and genomic microarrays.ConclusionWe show that a gene-desert region at 13q21 of approximately 3.9 Mb in size possesses an inherent potential to form evolutionarily new centromeres over, at least, approximately 95 million years of mammalian evolution. The striking absence of genes may represent an important property, making the region tolerant to the extensive pericentromeric reshuffling during subsequent evolution. Comparison of the pericentromeric organization of chromosome 13 in four Old World monkey species revealed many differences in sequence organization. The region contains clusters of duplicons showing peculiar features.

Epigenetic analysis of kinetochore assembly on variant human centromeres

Human centromere formation involves the assembly of the mitotic kinetochore onto chromosomal locations that contain the interphase prekinetochore. Immunofluorescent analysis of two functionally converse human centromere variants, neocentromeres and inactive centromeres, has been used to evaluate the functional significance of over 24 CENTROMERE proteins, providing important insight into the epigenetics of centromere formation and kinetochore assembly.

Esperanto for histones: CENP-A, not CenH3, is the centromeric histone H3 variant.

The first centromeric protein identified in any species was CENP-A, a divergent member of the histone H3 family that was recognised by autoantibodies from patients with scleroderma-spectrum disease. It has recently been suggested to rename this protein CenH3. Here, we argue that the original name should be maintained both because it is the basis of a long established nomenclature for centromere proteins and because it avoids confusion due to the presence of canonical histone H3 at centromeres.