Dissociation of minor satellite from the centromere in mouse

Dynamic of centromere associated RNAs and the centromere loading of DNA repair proteins in growing oocytes

Mammalian centromeres are generally composed of dispersed repeats and the satellites such as α-satellites in human and major/minor satellites in mouse. Transcription of centromeres by RNA polymerase II is evolutionary conserved and critical for kinetochore assembly. In addition, it has been found that the transcribed satellite RNAs can bind DNA repair proteins such as MRE11 and PRKDC, and excessively expressed satellite RNAs could induce genome instability and facilitate tumorigenesis. During the maturation of female oocyte, centromeres are critical for accurate segregation of homologous chromosomes and sister chromatids. However, the dynamics of oocyte centromere transcription and whether it associated with DNA repair proteins are unknown. In this study, we found the transcription of centromeres is active in growing oocytes but it is silenced when oocytes are fully grown. DNA repair proteins like Mlh1, Mre11 and Prkdc are found associated with the minor satellites and this association can be interfered by RNA polymerase II inhibitor α-amanitin. When the growing oocyte is in vitro matured, Mlh1/Mre11/Prkdc foci would release from centromeres to the ooplasm. If the oocytes are treated with Mre11 inhibitor Mirin, the meiosis resumption of growing oocytes with Mre11 foci can be suppressed. These data revealed the dynamic of centromeric transcription in oocytes and its potential association with DNA repair proteins, which provide clues about how oocytes maintain centromere stability and assemble kinetochores.

Evolution of long centromeres in fire ants

BACKGROUND

Centromeres are essential for accurate chromosome segregation, yet sequence conservation is low even among closely related species. Centromere drive predicts rapid turnover because some centromeric sequences may compete better than others during female meiosis. In addition to sequence composition, longer centromeres may have a transmission advantage.

RESULTS

We report the first observations of extremely long centromeres, covering on average 34 % of the chromosomes, in the red imported fire ant Solenopsis invicta. By comparison, cytological examination of Solenopsis geminata revealed typical small centromeric constrictions. Bioinformatics and molecular analyses identified CenSol, the major centromeric satellite DNA repeat. We found that CenSol sequences are very similar between the two species but the CenSol copy number in S. invicta is much greater than that in S. geminata. In addition, centromere expansion in S. invicta is not correlated with the duplication of CenH3. Comparative analyses revealed that several closely related fire ant species also possess long centromeres.

CONCLUSIONS

Our results are consistent with a model of simple runaway centromere expansion due to centromere drive. We suggest expanded centromeres may be more prevalent in hymenopteran insects, which use haplodiploid sex determination, than previously considered.

A critical evaluation of centromeric labeling to distinguish micronuclei induced by chromosomal loss and breakage in vitro

The in vitro micronucleus assay in conjunction with CREST-staining and fluorescence in situ hybridization (FISH) with centromere-specific DNA probes is being increasingly utilized for the detection of clastogenic and aneuploidy-inducing agents. Although potentially powerful techniques, both methods have unique characteristics that can influence sample processing and the interpretation of results. In this article, the use of the CREST and the FISH modifications of the in vitro micronucleus assay have been used to characterize the origin of the micronuclei induced by cyclophosphamide, 4,4'-methylene-bis(2-chloroaniline), 4-nitroquinoline N-oxide and ionizing radiation in metabolically competent MCL-5 cells or a derived cell line lacking metabolic activation. Using these results and our previous experiences with these techniques, a detailed comparison including the strengths and limitations of each technique as well as potential problems in performing each assay and in analyzing the data is discussed. In spite of their limitations, our results to date indicate that CREST-staining as well as FISH with centromere-specific DNA probes can be used to accurately distinguish micronuclei formed from chromosome loss from those originating from chromosome breakage and that these techniques can be valuable complements to the in vitro micronucleus assay.

Lack of recognition of a mouse centromere by anti-centromere proteins

In studies dealing with cancer as well as mutagenesis, the centromere region is being looked at critically. Generally, this region of chromosomes reacts to certain antibodies present in the sera of scleroderma patients. Some chromosomes in the Cl1D cell culture of mouse origin fail to respond to these antibodies even though, in a sub-metacentric chromosome, they do react at a site distal to the centromere. This site of reaction, though, coincides with the presence of minor satellite DNA. Is it that neither the minor satellite nor the so-called CENPs constitute an essential component of a functional centromere?

The centromeres of the Indian muntjac: evidence for the existence of multiple centromeres?

Unlike the centromeres of other species, the "compound' centromeres of the Indian muntjac span over exceptionally extended regions (Brinkley et al., 1984). We extend this concept and show that some of these centromeres are divisible into several chromomeres in which the light staining regions alternate with the dark staining C-band positive segments. Unlike the centromeres of other species where the centromere replicates as one unit, the replication of the sub-units constituting the centromere of the X-chromosome in the muntjac occurs at different times as at least three independent segments. The CREST staining of the centromere regions of even the smallest (Y2) chromosome is interrupted by non-staining segments. Electron microscopy shows similar interruptions in the continuity of the trilamellar kinetochore. Sister chromatid exchanges occur in the region of the centromeres and chromatid breaks within the centromere region occur in the non-fluorescent segments. We interpret these data to suggest that the centromere regions of the Indian muntjac are made up of independent multiple centromeres interrupted by non-centromeric chromatin. Relevance of these parameters in mutagenesis is briefly discussed.

Molecular-cytogenetic characterization of a higher plant centromere/kinetochore complex

The centromeric region of a telocentric field bean chromosome that resulted from centric fission of the metacentric satellite chromosome was microdissected. The DNA of this region was amplified and biotinylated by degenerate oligonucleotide-primed polymerase chain reaction (DOP-PCR)/linker-adapter PCR. After fluorescence in situ hybridization (FISH) the entire chromosome complement of Vicia faba was labelled by these probes except for the nucleolus organizing region (NOR) and the interstitial heterochromatin, the chromosomes of V. sativa and V. narbonensis were only slightly labelled by the same probes. Dense uniform labelling was also observed when a probe amplified from a clearly delimited microdissected centromeric region of a mutant of Tradescantia paludosa was hybridized to T. paludosa chromosomes. Even after six cycles of subtractive hybridization between DNA fragments amplified from centromeric and acentric regions no sequences specifically located at the field bean centromeres were found among the remaining DNA. A mouse antiserum was produced which detected nuclear proteins of 33 kDa and 68 kDa; these were predominantly located at V. faba kinetochores during mitotic metaphase. DNA amplified from the chromatin fraction adsorbed by this serum out of the sonicated total mitotic chromatin also did not cause specific labelling of primary constrictions. From these results we conclude: (1) either centromere-specific DNA sequences are not very conserved among higher plants and are - at least in species with large genomes - intermingled with complex dispersed repetitive sequences that prevent the purification of the former, or (2) (some of) the dispersed repeats themselves specify the primary constrictions by stereophysical parameters rather than by their base sequence.

Localization of anti-CENP antibodies and alphoid sequences in acentric heterochromatin in a breast cancer cell line

Karyotype alterations are a hallmark of cancer cells. Of particular interest to our laboratory are the inactive centromeres and blocks of heterochromatin devoid of the accompanying centromere. When purified or monospecies anticentromere proteins (CENP) antibodies or the whole serum from scleroderma (crest) patients are applied to human chromosomes, the centromere region exhibits the label. When we treated MDA 435 cells with the anti-CENP-A, anti-CENP-B, or the whole serum, the label was apparent on heterochromatin pericentric to the active and inactive centromeres. Moreover, blocks of heterochromatin not associated with any centromere also exhibited the label. Anti-CENP-C, however, is more strictly confined to the centromere in discrete dots and is not detected on any region except the sites of active centromeres. Distribution of alpha sequences also shows a pattern compatible with its distribution in the heterochromatin. Apparently, the use of anti-CENP-A and anti-CENP-B antibodies or alphoid DNA may not detect either the centromere (primary constriction) or the kinetochore; CENP-C may be an exception.