Native Centromeres Research Articles

An RNA Polymerase III-Dependent Heterochromatin Barrier at Fission Yeast Centromere 1

Heterochromatin formation involves the nucleation and spreading of structural and epigenetic features along the chromatin fiber. Chromatin barriers and associated proteins counteract the spreading of heterochromatin, thereby restricting it to specific regions of the genome. We have performed gene expression studies and chromatin immunoprecipitation on strains in which native centromere sequences have been mutated to study the mechanism by which a tRNAAlanine gene barrier (cen1 tDNAAla) blocks the spread of pericentromeric heterochromatin at the centromere of chromosome 1 (cen1) in the fission yeast, Schizosaccharomyces pombe. Within the centromere, barrier activity is a general property of tDNAs and, unlike previously characterized barriers, requires the association of both transcription factor IIIC and RNA Polymerase III. Although the cen1 tDNAAla gene is actively transcribed, barrier activity is independent of transcriptional orientation. These findings provide experimental evidence for the involvement of a fully assembled RNA polymerase III transcription complex in defining independent structural and functional domains at a eukaryotic centromere.

Comparison of Regional Brain Volume and Glucose Metabolism Between Patients with Mild Dementia with Lewy Bodies and Those with Mild Alzheimer's Disease

The aim of this study was to investigate regional differences between morphologic and functional changes in patients with mild dementia with Lewy bodies (DLB) compared with those with Alzheimer's disease (AD). Twenty patients with very mild DLB (mean age, 74.5 y; mean Mini-Mental State Examination [MMSE] score, 24.0), 20 patients with very mild AD (mean age, 74.1 y; mean MMSE score, 24.0), and 20 age- and sex-matched healthy volunteers (normal controls [NC]) underwent both (18)F-FDG PET and 3-dimensional spoiled gradient echo MRI. Fully automatic volumetry of the MRI data was used to obtain whole brain, hippocampal, occipital, and striatal volumes, which were compared with the results of a similar analysis of glucose metabolic data. In DLB patients, volumetric data indicated a significant volume decrease in the striatum, whereas (18)F-FDG PET showed significant glucose metabolic reductions in the temporal, parietal, and frontal areas--including in the occipital lobe--compared with those in the NC group. In contrast, in AD patients, both the hippocampal volume and glucose metabolism were significantly decreased, whereas the occipital volume and metabolism were preserved. Comparison of very mild DLB and AD revealed different morphologic and metabolic changes occurring in the medial temporal lobes and the occipital lobe, demonstrating characteristic pathophysiologic differences between these 2 diseases.

Genetic and Genomic Analysis of the AT-Rich Centromere DNA Element II of Saccharomyces cerevisiae

Centromere DNA element II (CDEII) of budding yeast centromeres is an AT-rich sequence essential for centromere (CEN) function. Sequence analysis of Saccharomyces cerevisiae CDEIIs revealed that A(5-7)/T(5-7) tracts are statistically overrepresented at the expense of AA/TT and alternating AT. To test the hypothesis that this nonrandom sequence organization is functionally important, a CEN library in which the CDEII sequences were randomized was generated. The library was screened for functional and nonfunctional members following centromere replacement in vivo. Functional CENs contained CDEIIs with the highly biased A(n)/T(n) run distribution of native centromeres, while nonfunctional CDEIIs resembled those picked from the library at random. Run content, defined as the fraction of residues present in runs of four or more nucleotides, of the functional and nonfunctional CDEII populations differed significantly (P < 0.001). Computer searches of the genome for regions with an A + T content comparable to CDEIIs revealed that such loci are not unique to centromeres, but for 14 of the 16 chromosomes the AT-rich locus with the highest A(n > or =4) + T(n > or =4) run content was the centromere. Thus, the distinctive and nonrandom sequence organization of CDEII is important for centromere function and possesses informational content that could contribute to the determination of centromere identity.

CENP-C is necessary but not sufficient to induce formation of a functional centromere.

CENP-C is an evolutionarily conserved centromeric protein. We have used the chicken DT40 cell line to test the idea that CENP-C is sufficient as well as necessary for the formation of a functional centromere. We have compared the effects of disrupting the localization of CENP-C with those of inducibly overexpressing the protein. Removing CENP-C from the centromere causes disassembly of the centromere protein complex and blocks cells at the metaphase-anaphase junction. Overexpressed CENP-C is associated with an increase in errors of chromosome segregation and inhibits the completion of mitosis. However, the excess CENP-C does not disrupt the native centromeres detectably and does not associate with another conserved centromere protein, ZW10. The distribution of the excess CENP-C changes during the cell cycle. In metaphase, the excess CENP-C coats the chromosome arms. At the metaphase-anaphase transition, the excess CENP-C clusters, and during interphase it is present in large bodies which form around pre-existing centromeres which are also clustered. These results indicate that CENP-C is necessary but not sufficient for the formation of a functional centromere and suggest that the structure of CENP-C may be regulated during the cell cycle.

Anticentromere autoantibodies in patients without Raynaud's disease or systemic sclerosis.

Anticentromere autoantibodies (ACA) are associated with Raynaud's disease and systemic sclerosis (SSc). ACA usually bind at least one of three major centromere proteins (CENPs), particularly CENP-B. We identified 16 patients with ACA who do not have Raynaud's disease or SSc. The objective of this study was to determine whether these 16 ACA differ in antigenic specificity from the ACA found in patients with Raynaud's disease or SSc. Binding of these serum ACA was tested using competition experiments with recombinant CENP-B, and native centromere proteins from HEp-2 cells and HeLa nuclear extracts in ELISAs, immunoblots, and indirect immunofluorescence assays. The ACA from these 16 patients are strikingly different from those obtained from patients who have Raynaud's disease or SSc. Only 5 of the 16 index sera (31.25%) bound CENP-B from two or more different sources by at least two methods. Six of these 16 sera (37.5%) did not bind CENP-B on ELISA, and 8 of 16 (43.75%) did not bind CENP-B on immunoblots. Three sera did not bind CENP-B either by ELISA or immunoblots. Of the 13 sera that bound CENP-B, their patterns of binding to CENP-B strongly suggested that they bind different epitopes within the CENP-B antigen. Independently of their binding to CENP-B, these sera reacted mainly with minor CENP antigens detected by HeLa nuclear extracts. We have identified unusual ACA not associated with Raynaud's disease or SSc.

The centromere enhancer mediates centromere activation in Schizosaccharomyces pombe.

The centromere enhancer is a functionally important DNA region within the Schizosaccharomyces pombe centromeric K-type repeat. We have previously shown that addition of the enhancer and cen2 centromeric central core to a circular minichromosome is sufficient to impart appreciable centromere function. A more detailed analysis of the enhancer shows that it is dispensable for centromere function in a cen1-derived minichromosome containing the central core and the remainder of the K-type repeat, indicating that the critical centromeric K-type repeat, like the central core, is characterized by functional redundancy. The centromeric enhancer is required, however, for a central core-carrying minichromosome to exhibit immediate centromere activity when the circular DNA is introduced via transformation into S. pombe. This immediate activation is probably a consequence of a centromere-targeted epigenetic system that governs the chromatin architecture of the region. Moreover, our studies show that two entirely different DNA sequences, consisting of elements derived from two native centromeres, can display centromere function. An S. pombe CENP-B-like protein, Abp1p/Cbp1p, which is required for proper chromosome segregation in vivo, binds in vitro to sites within and adjacent to the modular centromere enhancer, as well as within the centromeric central cores. These results provide direct evidence in fission yeast of a model, similar to one proposed for mammalian systems, whereby no specific sequence is necessary for centromere function but certain classes of sequences are competent to build the appropriate chromatin foundation upon which the centromere/kinetochore can be formed and activated.

Lambda CM8, a human sequence with putative centromeric function, does not map to the centromere but is present in one to two copies at 9qter.

A DNA fragment isolated from a human genomic library, was reported to be present at all human centromeres and present at 16-32 copies per genome. Reintroduction of this DNA into mammalian cells as a concatenated phage clone gave rise to dicentric chromosomes which gave rise to a new, stable, chromosome. Taken together these observations could mean that this DNA is part of a native centromere. We have reexamined the location and copy number of this sequence and find it to be present at 1-2 copies per genome with a single site of in situ hybridisation at 9qter.

The higher order structure of the centromere

The architecture of the centromere region of mouse chromosomes has been studied in cells grown in the presence of 5-azacytidine. This drug interferes with normal condensation producing elongated centromere regions. It has been found that this effect is reversible in the presence of the drug, allowing the observation of the repackaging of the extended centromere into a structure exhibiting native centromere morphology. Light microscopy as well as transmission and scanning electron microscopy of this condensation process suggests that the native centromere is formed by the helical folding of a subfiber with an approximate diameter of 100 nm. This fiber is in turn composed of loops of the 30-nm fiber class. The boundary between successive gyres of the subfiber are obscured at the completion of condensation resulting in the formation of a homogenous 250- to 300-nm fiber that is the native centromere. These observations provide evidence for an additional level of chromatin organization within the metaphase chromosome.