Indirect immunofluorescence of inactive centromeres as indicator of centromeric function

Anti-CENP-C Antibody-Based Immunofluorescence Dicentric Assay: Radiation Dose-Response, Validation Studies, and Radiation Dose-Dependency on Sister Centromere Fluorescence

Dicentric chromosome assay (DCA) is the most accepted cytological technique for the purpose of biological dosimetry in radiological and nuclear accidents, however, it is not always easy to evaluate dicentric chromosomes because of the technical difficulty in identifying dicentric chromosomes on Giemsa-stained metaphase chromosome samples. Here, we applied an antibody recognizing centromere protein (CENP) C, CENP-C, whose antigenicity is resistant to the fixation with Carnoy's solution. Normal human diploid cells were irradiated with various doses of 137Cs γ rays at 1 Gy/ min, treated with hypotonic solution, fixed with Carnoy's fixative, and metaphase chromosome spreads were stained with anti-CENP-C antibody. Dose-dependent induction of dicentric chromosomes was confirmed between 1 and 10 Gy of γ rays, and the results were compatible with those obtained by the conventional Giemsa-stained chromosome samples. The CENP-C assay also uncovered the difference in the fluorescence from the sister centromeres on the same chromosome, which was more pronounced after radiation exposure. Although the underlying mechanism is still to be determined, the result suggests a novel effect of radiation on centromeres. The innovative protocol for CENP-C-based DCA, which enables ideal visualization of centromeres, is simple, effective and reliable. It does not require skilled examiners, so that it may be an alternative method, avoiding uneasiness of the current DCA using Giemsa-stained metaphase chromosome samples.

Chromosome Segregation Is Biased by Kinetochore Size

Chromosome missegregation during mitosis or meiosis is a hallmark of cancer and the main cause of prenatal death in humans. The gain or loss of specific chromosomes is thought to be random, with cell viability being essentially determined by selection. Several established pathways including centrosome amplification, sister-chromatid cohesion defects, or a compromised spindle assembly checkpoint can lead to chromosome missegregation. However, how specific intrinsic features of the kinetochore—the critical chromosomal interface with spindle microtubules—impact chromosome segregation remains poorly understood. Here we used the unique cytological attributes of female Indian muntjac, the mammal with the lowest known chromosome number (2n = 6), to characterize and track individual chromosomes with distinct kinetochore size throughout mitosis. We show that centromere and kinetochore functional layers scale proportionally with centromere size. Measurement of intra-kinetochore distances, serial-section electron microscopy, and RNAi against key kinetochore proteins confirmed a standard structural and functional organization of the Indian muntjac kinetochores and revealed that microtubule binding capacity scales with kinetochore size. Surprisingly, we found that chromosome segregation in this species is not random. Chromosomes with larger kinetochores bi-oriented more efficiently and showed a 2-fold bias to congress to the equator in a motor-independent manner. Despite robust correction mechanisms during unperturbed mitosis, chromosomes with larger kinetochores were also strongly biased to establish erroneous merotelic attachments and missegregate during anaphase. This bias was impervious to the experimental attenuation of polar ejection forces on chromosome arms by RNAi against the chromokinesin Kif4a. Thus, kinetochore size is an important determinant of chromosome segregation fidelity.

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Centromere/kinetochore functional layers scale proportionally with centromere size

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Kinetochore microtubule binding capacity scales with kinetochore size

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Chromosome congression and bi-orientation are biased by kinetochore size

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Error formation leading to chromosome missegregation is biased by kinetochore size

Centromere activity in dicentric small supernumerary marker chromosomes

Twenty-five dicentric small supernumerary marker chromosomes (sSMC) derived from #13/21, #14, #15, #18, and #22 were studied by immunohistochemistry for their centromeric activity. Centromere protein (CENP)-B was applied as marker for all centromeres and CENP-C to label the active ones. Three different 'predominant' activation patterns could be observed, i.e., centric fusion or either only one or all two centromeres were active. In one inherited case, the same activation pattern was found in mother and son. In acrocentric-derived sSMC, all three activation patterns could be present. In contrary, in chromosome 18-derived sSMC, only the fusion type was observed. In concordance with previous studies a certain centromeric plasticity was observed in up to 13% of the cells of an individual case. Surprisingly, the obtained data suggests a possible influence of the sSMC carrier's gender on the implementation of the predominant activation pattern; especially, only one active centromere was found more frequently in female than in male carriers. Also, it might be suggested that dicentric sSMC with one active centromere could be less stable than such with two active ones-centromeric plasticity might have an influence here, as well. Also, centromere activity in acrocentric-derived dicentrics could be influenced by heteromorphisms of the corresponding short arms. Finally, evidence is provided that the closer the centromeres of a dicentric are and if they are not fused, the more likely it was that both of them became active. In concordance and refinement with previous studies, a distance of 1.4 Mb up to about 13 Mb the two active centromere state was favored, while centromeric distance of over approximately 15 Mb lead to inactivation of one centromere. Overall, here, the first and largest ever undertaken study in dicentric sSMC is presented, providing evidence that the centromeric activation pattern is, and parental origin may be of interest for their biology. Influence of mechanisms similar or identical to meiotic imprinting in the centromeric regions of human chromosomes might be present. Furthermore, centromeric activation pattern could be at least in parts meaningful for the clinical outcome of dicentric sSMC, as sSMC stability and mosaicism can make the difference between clinically normal and abnormal phenotypes.

Chromosome size and origin as determinants of the level of CENP-A incorporation into human centromeres

We have expressed an EGFP-CENP-A fusion protein in human cells in order to quantitate the level of CENP-A incorporated into normal and variant human centromeres. The results revealed a 3.2-fold difference in the level of CENP-A incorporation into alpha-satellite repeat DNA-based centromeres, with the Y centromere showing the lowest level of all normal human chromosomes. Identification of individual chromosomes revealed a statistically significant, though not absolute, correlation between chromosome size and CENP-A incorporation. Analysis of three independent neocentromeres revealed a significantly reduced level of CENP-A compared to normal centromeres. Truncation of a neocentric marker chromosome to produce a minichromosome further reduced CENP-A levels, indicating a remodelling of centromeric chromatin. These results suggest a role for increased CENP-A incorporation in the faithful segregation of larger chromosomes and support a model of centromere evolution in which neocentromeres represent ancestral centromeres that, through adaptive evolution, acquire satellite repeats to facilitate the incorporation of higher numbers of CENP-A containing nucleosomes, thereby facilitating the assembly of larger kinetochore structures.

Chromosome stability is maintained by short intercentromeric distance in functionally dicentric human Robertsonian translocations

While the formation of a dicentric chromosome often leads to chromosome instability, human dicentric Robertsonian translocations usually remain stable. To investigate the basis for this stability, we have examined the centromeres of 15 structurally dicentric rob(13q14q) Robertsonian translocations using immunofluorescence and fluorescence in situ hybridization (FISH). The immunofluorescence detection of centromere protein C (CENP-C) was used as a marker for centromere function as CENP-C seems to play an essential role in kinetochore structure and stability and was previously shown to be absent from inactive centromeres. In all 15 translocation-containing cell lines, CENP-C was confined to only one of the centromeres of the translocation in a fraction of the cells analyzed. This suggests that centromere inactivation commonly occurs on dicentric Robertsonian translocations and may serve as one mechanism allowing for their stability. However, in the majority of the translocations (12 out of 15), a portion of the cells analyzed displayed CENP-C immunofluorescence at both centromeres, suggesting that both centromeres were active and that the translocation was functionally dicentric. The percentage of cells with CENP-C at both centromeres ranged from 2% to 82%. These results support the hypothesis that the close proximity of two functional centromeres on Robertsonian translocations allows them to remain stable.

Epigenetic control of mammalian centromere protein binding: does DNA methylation have a role?

Chromosome 1 of the inbred mouse strain DBA/2 has a polymorphism associated with the minor satellite DNA at its centromere. The more terminal block of satellite DNA sequences on this chromosome acts as the centromere as shown by the binding of CREST ACA serum, anti-CENP-B and anti-CENP-E polyclonal sera. Demethylation of the minor satellite DNA sequences accomplished by growing cells in the presence of the drug 5-aza-2′-deoxycytidine results in a redistribution of the CENP-B protein. This protein now binds to an enlarged area on the more terminal block and in addition it now binds to the more internal block of minor satellite DNA sequences on chromosome 1. The binding of the CENP-E protein does not appear to be affected by demethylation of the minor satellite sequences. We present a model to explain these observations. This model may also indicate the mechanism by which the CENP-B protein recognises specific sites within the arrays of minor satellite DNA on mouse chromosomes.

Structural instability of a transmissible end-to-end dicentric chromosome in a xeroderma pigmentosum fibroblast clone

Cytogenetic analysis was performed in a fibroblast clone (XP9UV25) selected for anchorage-independent growth from an XP strain of normal origin and characterized by the presence of clonal chromosome rearrangements. A dicentric chromosome involving the 5p and 16q telomeric regions was observed in XP9UV25 cells at the fifth passage from colony isolation and at successive passages. The specific anomaly was present with increasing frequency (from 22 to 60% of mitoses) during culture propagation, undergoing rearrangements that gave rise to: 1) (5;16) dicentrics with deletions or duplications of the intercentromeric region; 2) homodicentrics for chromosomes 5 or 16, either end-to-end associations or rearranged; and 3) derivative 5p+ and 16q+ monocentric chromosomes. The frequency of other anomalies involving other chromosomes was negligible. These findings represent the first demonstration that a telomeric association leads to a variety of balanced and unbalanced chromosome rearrangements. These rearrangements may result from asymmetric interchanges between sister chromatids, "bridge-breakage-fusion" events during cell division, breakage and reunion of isochromatids, and breakage followed by healing of the ends. The type of anomaly and the sequence of karyotypic changes we observed in the XP9UV25 clone and their mechanisms of origin may be the same as those occurring during transformation from diploidy to aneuploidy in neoplastic cells.

A stable dicentric chromosome: both centromeres develop kinetochores and attach to the spindle in monocentric and dicentric configuration

A stable, dicentric human chromosome, which is known from light microscopy to show a 50:50 distribution between monocentric/dicentric appearance, was examined by conventional electron microscopy and after labelling the centromere with anticentromere antibodies from CREST serum. Both centromeres of the chromosome developed kinetochores whether in monocentric or dicentric configuration. The eight monocentrics observed had all developed kinetochores at the centromere outside the constriction; at least six of them also had kinetochores at the centromere in the constriction. The dicentrics from glutaraldehyde fixed cells had spindle microtubules attached to both kinetochore sets irrespective of monocentric/dicentric configuration. The chromosome thus appeared to use both centromeres, either equally or with one serving a chromatid adhesion function while the second was used for transport along the spindle.

On the mode of evolution of alpha satellite DNA in human populations

The hypothesis that highly reiterated satellite DNAs in present-day populations evolve by molecular mechanisms that create, by saltatory amplification steps, new long arrays of satellite DNA, and that such long arrays are used for homogenization purposes, has been tested both in mouse and in humans. In mouse, the data obtained are consistent with this hypothesis. This was tested in more detail on chromosomes 13 and 21 of the human genome. A Centre d'Etudes du Polymorphisme Humain family, which in some individuals exhibits strong supplementary DNA bands following TaqI restriction endonuclease digestion and conventional gel electrophoresis, was analyzed by pulse field gel electrophoresis following restriction by BamHI. The supplementary bands on chromosome 13 (18 times the basic alpha satellite DNA repeat) and on chromosome 21 (a 9.5-mer) segregated with centromeric alpha satellite DNA blocks of 5 and 5.3 megabases, respectively. These are by far the largest alpha satellite block lengths seen in all chromosome 13 and chromosome 21 centromeric sequences so far analyzed in this manner. The possibility that these supplementary alpha satellite sequences were created in single individuals by saltatory amplification steps is discussed in light of our own data and that published by others. It is proposed that deletion events and unequal cross-overs, which both occur in large satellite DNA arrays, contribute to the homogenization of size and sequence of the alpha satellite DNA on most chromosomes of humans.

Micronuclei with kinetochores in human melanoma cells and rectal carcinomas

Micronucleus frequencies were analysed in an X-irradiated human melanoma cell line and in untreated rectal carcinoma cells. As a special aspect of the micronucleus formation, micronuclei-containing kinetochores were analysed by the method of indirect immunofluorescence. The incidence of kinetochore-positive micronuclei was taken as a measure of chromosome loss. In order to show kinetochores, an anti-kinetochore serum of a CREST syndrome patient was used. In irradiated melanoma cells the mean number of micronuclei increased linearly with increasing dose whereas the proportion of kinetochore-positive micronuclei decreased. The analysis of the time-dependent formation of micronuclei revealed an increase of the micronucleus frequency from 24 to 48 h after irradiation (p.r.) and a slight decrease from 48 to 72 h p.r. The highest number of kinetochore-containing micronuclei was scored 72 h p.r. In rectal carcinoma cells a high rate of spontaneously formed micronuclei was observed. At least 30% of these micronuclei contained complete chromosomes, as indicated by the kinetochore-positive staining. We have attempted to summarize the data by means of a numerical expression which takes into account the relation between lost chromosomes, visible as kinetochore-positive micronuclei, and the total sum of micronuclei.

Partial deletion of alpha satellite DNA associated with reduced amounts of the centromere protein CENP-B in a mitotically stable human chromosome rearrangement

A familial, constitutionally rearranged human chromosome 17 is deleted for much of the DNA in its centromeric region but retains full mitotic centromere activity. Fluorescence in situ hybridization, pulsed-field gel electrophoresis, and Southern blot analysis of the residual centromeric region revealed a approximately 700-kb centromeric array of tandemly repeated alpha satellite DNA that was only approximately 20 to 30% as large as a normal array. This deletion was associated with a reduction in the amount of the centromere-specific antigen CENP-B detected by indirect immunofluorescence. The coincidence of the primary constriction, the small residual array of alpha satellite DNA, and the reduced amount of detectable CENP-B support the hypothesis that CENP-B is associated with alpha satellite DNA. Furthermore, the finding that both the deleted chromosome 17 and its derivative supernumerary fragment retained mitotic function and possess centromeric protein antigens suggests that human centromeres are structurally and functionally repetitive.

Partial deletion of alpha satellite DNA associated with reduced amounts of the centromere protein CENP-B in a mitotically stable human chromosome rearrangement

A familial, constitutionally rearranged human chromosome 17 is deleted for much of the DNA in its centromeric region but retains full mitotic centromere activity. Fluorescence in situ hybridization, pulsed-field gel electrophoresis, and Southern blot analysis of the residual centromeric region revealed a approximately 700-kb centromeric array of tandemly repeated alpha satellite DNA that was only approximately 20 to 30% as large as a normal array. This deletion was associated with a reduction in the amount of the centromere-specific antigen CENP-B detected by indirect immunofluorescence. The coincidence of the primary constriction, the small residual array of alpha satellite DNA, and the reduced amount of detectable CENP-B support the hypothesis that CENP-B is associated with alpha satellite DNA. Furthermore, the finding that both the deleted chromosome 17 and its derivative supernumerary fragment retained mitotic function and possess centromeric protein antigens suggests that human centromeres are structurally and functionally repetitive.

Centromeres of mammalian chromosomes

The centromere is the major cis-acting genetic locus involved in chromosome segregation in mitosis and meiosis. The mammalian centromere is characterized by large amounts of tandemly repeated satellite DNA and by a number of specific centromere proteins, at least one of which has been shown to interact directly with centromeric satellite DNA sequences. Although direct functional assays of chromosome segregation are still lacking, the data are most consistent with a structural and possibly functional role for satellite DNA in the mammalian centromere.

Deletion of specific sequences or modification of centromeric chromatin are responsible for Y chromosome centromere inactivation

Stable dicentric chromosomes behave as monocentrics because one of the centromeres is inactive. The cause of centromere inactivation is unknown; changes in centromere chromatin conformation and loss of centromeric DNA elements have been proposed as possible mechanisms. We studied the phenomenon of inactivation in two Y centromeres, having as a control genetically identical active Y centromeres. The two cases have the following karyotypes: 45, X/46,X,i(Y)(q12) and 46,XY/47,XY,+t(X;Y) (p22.3;p11.3). The analysis of the behavior of the active and inactive Y chromosome centromeres after Da-Dapi staining, CREST immunofluorescence, and in situ hybridization with centromeric probes leads us to conclude that, in the case of the isochromosome, a true deletion of centromeric chromatin is responsible for its stability, whereas in the second case, stability for its stability, whereas in the second case, stability of the dicentric (X;Y) is the result of centromere chromatin modification.

Alternate centromere inactivation in a pseudodicentric (15;20)(pter;pter) associated with a progressive neurological disorder

A 13 year old male with a severe progressive neurological disorder was found to have a pseudodicentric chromosome resulting from a telomeric fusion 15p;20p. In lymphocytes, the centromeric constriction of the abnormal chromosome was always that of the chromosome 20, while in fibroblasts both centromeres were alternately constricted. Cd staining was positive only at the active centromere, but a weak anticentromere immunofluorescence was present at the inactive one. We suggest that centromere inactivation results from a modified conformation of the functional DNA sequences preventing normal binding to centromere specific proteins. We also postulate that the patient's disorder, reminiscent of a spongy glioneuronal dystrophy as seen in Alper's and Creutzfeldt-Jakob diseases, may be secondary to the presence of the pathogenic isoform of the prion protein encoded by a gene mapped to 20p12----pter.

Visualization of centromere proteins CENP-B and CENP-C on a stable dicentric chromosome in cytological spreads

We have screened for the presence of two centromere autoantigens, CENP-B (80 kDa) and CENP-C (140 kDa) at the inactive centromere of a naturally occurring stable dicentric chromosome using specific antibodies that do not cross-react with any other chromosomal proteins. In order to discriminate between the active and inactive centromeres on this chromosome we have developed a modification of the standard methanol/acetic acid fixation procedure that allows us to obtain high-quality cytological spreads that retain antigenicity with the anti-centromere antibodies. We have noted three differences in the immunostaining patterns with specific anti-CENP-B and CENP-C antibodies. (1) The amount of detectable CENP-B varies from chromosome to chromosome. The amount of CENP-C appears to be more or less the same on all chromosomes. (2) CENP-B is present at both active and inactive centromeres of stable dicentric autosomes. CENP-C is not detectable at the inactive centromeres. (3) While immunofluorescence with anti-CENP-C antibodies typically gives two discrete spots, staining with anti-CENP-B often appears as a single bright bar connecting both sister centromeres. This suggests that while CENP-C may be confined to the outer centromere in the kinetochore region, CENP-B may be distributed throughout the entire centromere. Our data suggest that CENP-C is likely to be a component of some invariant chromosomal substructure, such as the kinetochore. CENP-B may be involved in some other aspect of centromere function, such as chromosome movement or DNA packaging.

Kinetochore development in two dicentric chromosomes in man. A light and electron microscopic study

Two dicentric human chromosomes were investigated with light and electron microscopic techniques. One chromosome, with a translocation tdic(5;13)(p12;p12), behaved as a dicentric in about half the cells: it had two primary constrictions; C- and Cd-banding showed two centromeres; and the CREST antikinetochore antibody reacted with the two centromeres with equal affinity. Electron microscopic analysis of sectioned metaphases showed that the dicentric could develop kinetochores at both centromeres simultaneously. The other dicentric chromosome, tdic(21;21)(q22;q22), occasionally showed two primary constrictions, but both C- and Cd-banding distinguished between an active and an inactive centromere, and the CREST antibody reacted only weakly with the inactive centromere. Electron microscopy showed kinetochore development at only one centromere.

Flow cytometry measurements of human chromosome kinetochore labeling

A method for the preparation and measurement of immunofluorescent human chromosome centromeres in suspension is described using CREST antibodies, which bind to the centromeric region of chromosomes. Fluorescein isothiocyanate (FITC)-conjugated antihuman antibodies provide the fluorescent label. Labeled chromosomes are examined on microscope slides and by flow cytometry. In both cases a dye which binds to DNA is added to provide identification of the chromosome groups. Sera from different CREST patients vary in their ability to bind to chromosome arms in addition to the centromeric region. Flow cytometry and microfluorimetry measurements have shown that with a given CREST serum the differences in kinetochore fluorescence between chromosomes are only minor. Flow cytometry experiments to relate the number of dicentric chromosomes, induced by in vitro radiation of peripheral blood cells to the slightly increased number of chromosomes with above-average kinetochore fluorescence did not produce decisive radiation dosimetry results.

Familial occurrence of isodicentric X chromosomes with different breakpoints

We report two cases of an idic (X) chromosome found in relatives with Turner's syndrome. A 21-year-old female revealed a non-mosaic form of X isochromosome of the long arms with two C-band regions, i.e. dic(X)(qter----cen----p11::p11----cen----qter). Her 46-year-old aunt with Turner's syndrome had an X chromosome with long arm breakpoints at site q21 and chromosomal mosaicism, i.e. 45,X/46,X, dic(X)(pter----q21::q21----pter)(78/22). The relative rarity of reports about familial Turner's syndrome with structural abnormality may suggest a coincidence. However, it is difficult to exclude familial predisposition to X isochromosome formation in this family.

Analysis of double minutes and double minute-like chromatin in human and murine tumor cells using antikinetochore antibodies

Antikinetochore antibodies from patients with the calcinosis, Raynaud's phenomenon, esophageal dismobility, sclerodactyly, telangiectasia-(CREST)-syndrome of scleroderma were used as immunofluorescent probes to discriminate between the presence and absence of kinetochores in minute chromosomes not previously seen by conventional banding methods. Double minute chromosomes (DM) consistently lack the antigenic component of the kinetochore, which is direct evidence for the fact that they do not have a centromere. Although somatically stable in malignant cell populations, DM are unable to attach to the mitotic spindle. Conversely, despite their structural similarity to DM, chromosome fragments and supernumerary marker chromosomes exhibit intensely fluorescing kinetochores and, thus, are subject to a precise anaphasic distribution.

Centromeric proteins recognized by CREST sera and meiotic chromosome segregation

Analysis of the peptides recognized by CREST sera was carried out in different mouse tissues and cells, including spermatozoa. In all cases, a polypeptide of Mr = 18,000 was recognized by the sera and occasionally two other proteins of Mr = 80,000 and Mr = 140,000 were observed after immunoblotting of nuclear proteins. In both early and late spermatids, centromeric staining was observed after incubation and immunofluorescence with CREST sera. After detergent treatment, it was even possible to detect centromeric staining in mature spermatozoa. In spermatid cells, the immunofluorescent pattern presented a binomial distribution of the number of fluorescent spots, with a mean value around half of the haploid number of chromosomes. Since this pattern is the result of chromosome segregation after meiosis II, our data suggest that this centromeric peptide is not directly implicated in the chromosome segregation process. On the other hand, the distribution of spots after immunofluorescence suggests a different organization of centromeric components in meiosis I and meiosis II.

A technique for simultaneous antikinetochore immunofluorescence staining and Q-banding in chromosomes from human lymphocytes

Antikinetochore immunofluorescence staining has been used in several studies to determine whether a second kinetochore is present, active, or both, in multicentric chromosomes. All of these studies have used tissue culture cells, and contended with the problem of obtaining well spread, banded metaphase chromosomes without affecting the kinetochore staining. We have adapted hypotonic, centrifugation and chromosome staining techniques to obtain simultaneous Q-banding and bright kinetochore staining of chromosomes from human lymphocytes.

Molecular cloning of cDNA for CENP-B, the major human centromere autoantigen

We have isolated a series of overlapping cDNA clones for approximately 95% of the mRNA that encodes CENP-B, the 80-kD human centromere autoantigen recognized by patients with anticentromere antibodies. The cloned sequences encode a polypeptide with an apparent molecular mass appropriate for CENP-B. This polypeptide and CENP-B share three non-overlapping epitopes. The first two are defined by monoclonal antibodies elicited by injection of cloned fusion protein. Epitope 1 corresponds to a major antigenic site recognized by the anticentromere autoantibody used to obtain the original clone. Epitope 2 is a novel one not recognized by the autoantibody. These epitopes were shown to be distinct both by competitive binding experiments and by their presence or absence on different subcloned portions of the fusion protein. The third independent epitope, recognized by a subset of anticentromere-positive patient sera, maps to a region substantially closer to the amino terminus of the fusion protein. DNA and RNA blot analyses indicate that CENP-B is unrelated to CENP-C, a 140-kD centromere antigen also recognized by these antisera. CENP-B is the product of a 2.9-kb mRNA that is encoded by a single genetic locus. This mRNA is far too short to encode a polypeptide the size of CENP-C. The carboxy terminus of CENP-B contains two long domains comprised almost entirely of glutamic and aspartic acid residues. These domains may be responsible for anomalous migration of CENP-B on SDS-polyacrylamide gels, since the true molecular mass of CENP-B is approximately 65 kD, 15 kD less than the apparent molecular mass deduced from gel electrophoresis. Quite unexpectedly, immunofluorescence analysis using antibodies specific for CENP-B reveals that the levels of antigen vary widely between chromosomes.

Size variation in kinetochores of human chromosomes

Aneuploidy, the loss or gain of chromosomes from cells, is likely in many cases to involve the kinetochore, the site of attachment of spindle microtubules. We analyzed human fibroblast cells with antikinetochore-antibody indirect immunofluorescence, and noted an apparent heterogeneity in the sizes of kinetochores among different chromosomes. The Y chromosome in particular always showed minute kinetochores, an observation which was quantified and substantiated using computer-assisted image analysis. This finding, combined with literature reports about in vivo and in vitro involvement of the Y chromosome in aneuploidy, was used to frame a novel hypothesis about the generation of chromosome imbalance.