Chromosomal dynamics of human neocentromere formation

Multiple pathways can bypass the essential role of fission yeast Hsk1 kinase in DNA replication initiation

A number of different genetic backgrounds and growth conditions bypass DNA replication defects caused by the absence of yeast Hsk1 kinase, demonstrating the plasticity of the eukaryotic DNA replication program.

Cdc7/Hsk1 is a conserved kinase required for initiation of DNA replication that potentially regulates timing and locations of replication origin firing. Here, we show that viability of fission yeast hsk1Δ cells can be restored by loss of mrc1, which is required for maintenance of replication fork integrity, by cds1Δ, or by a checkpoint-deficient mutant of mrc1. In these mutants, normally inactive origins are activated in the presence of hydroxyurea and binding of Cdc45 to MCM is stimulated. mrc1Δ bypasses hsk1Δ more efficiently because of its checkpoint-independent inhibitory functions. Unexpectedly, hsk1Δ is viable at 37°C. More DNA is synthesized, and some dormant origins fire in the presence of hydroxyurea at 37°C. Furthermore, hsk1Δ bypass strains grow poorly at 25°C compared with higher temperatures. Our results show that Hsk1 functions for DNA replication can be bypassed by different genetic backgrounds as well as under varied physiological conditions, providing additional evidence for plasticity of the replication program in eukaryotes.

Formation of novel CENP-A domains on tandem repetitive DNA and across chromosome breakpoints on human chromosome 8q21 neocentromeres

Endogenous human centromeres form on megabase-sized arrays of tandemly repeated alpha satellite DNA. Human neocentromeres form epigenetically at ectopic sites devoid of alpha satellite DNA and permit analysis of centromeric DNA and chromatin organization. In this study, we present molecular cytogenetic and CENP-A chromatin immunoprecipitation (ChIP) on CHIP analyses of two neocentromeres that have formed in chromosome band 8q21 each with a unique DNA and CENP-A chromatin configuration. The first neocentromere was found on a neodicentric chromosome 8 with an inactivated endogenous centromere, where the centromeric activity and CENP-A domain were repositioned to band 8q21 on a large tandemly repeated DNA. This is the first example of a neocentromere forming on repetitive DNA, as all other mapped neocentromeres have formed on single copy DNA. Quantitative fluorescent in situ hybridization (FISH) analysis showed a 60% reduction in the alpha satellite array size at the inactive centromere compared to the active centromere on the normal chromosome 8. This neodicentric chromosome may provide insight into centromere inactivation and the role of tandem DNA in centromere structure. The second neocentromere was found on a neocentric ring chromosome that contained the 8q21 tandemly repeated DNA, although the neocentromere was localized to a different genomic region. Interestingly, this neocentromere is composed of two distinct CENP-A domains in bands 8q21 and 8q24, which are brought into closer proximity on the ring chromosome. This neocentromere suggests that chromosomal rearrangement and DNA breakage may be involved in neocentromere formation. These novel examples provide insight into the formation and structure of human neocentromeres.

Heterochromatin boundaries are hotspots for de novo kinetochore formation

The centromere-specific histone H3 variant CENH3 (also known as CENP-A) is considered to be an epigenetic mark for establishment and propagation of centromere identity. Pulse induction of CENH3 (Drosophila CID) in Schneider S2 cells leads to its incorporation into non-centromeric regions and generates CID islands that resist clearing from chromosome arms for multiple cell generations. We demonstrate that CID islands represent functional ectopic kinetochores, which are non-randomly distributed on the chromosome and show a preferential localization near telomeres and pericentric heterochromatin in transcriptionally silent, intergenic chromatin domains. Although overexpression of heterochromatin protein 1 (HP1) or increasing histone acetylation interferes with CID island formation on a global scale, induction of a locally defined region of synthetic heterochromatin by targeting HP1-LacI fusions to stably integrated Lac operator arrays produces a proximal hotspot for CID deposition. These data indicate that the characteristics of regions bordering heterochromatin promote de novo kinetochore assembly and thereby contribute to centromere identity.

Interstitial deletion of proximal 8q including part of the centromere from unbalanced segregation of a paternal deletion/marker karyotype with neocentromere formation at 8p22

BACKGROUND/AIMS

The 'McClintock mechanism' of chromosome breakage and centromere misdivision, in which a deleted chromosome with its concomitant excised marker or ring chromosome is formed, has been described in approximately one dozen reports. We report a case of a girl with short stature, developmental delay, and dysmorphic features.

METHODS

Analysis was performed on the proband and father using cytogenetic chromosome analysis and the Affymetrix 6.0 SNP microarray. Fluorescence in situ hybridization (FISH) using a chromosome 8 alpha-satellite probe and immunofluorescence with antibodies to CENP-C were used to examine the centromere positions in these chromosomes.

RESULTS

An abnormal chromosome 8 with a cytogenetically visible deletion was further defined by SNP array as a 10.6-Mb deletion from 8q11.1→q12.1. FISH with a chromosome 8 alpha-satellite probe demonstrated that the deletion removed a significant portion of the pericentromeric alpha-satellite repeat sequences and proximal q arm. The deleted chromosome 8 appeared to have a constriction at 8p22, suggesting the formation of a neocentromere, even though alpha-satellite sequences still appeared at the normal location. Chromosome analysis of the phenotypically normal father revealed the same deleted chromosome 8, as well as an apparently balancing mosaic marker chromosome 8. FISH studies revealed that the majority of the chromosome 8 alpha-satellite DNA resided in the marker chromosome. Immunofluorescence studies with antibodies to CENP-C, a kinetochore protein, proved the presence of a neocentromere at 8p22. The excision of the marker from the deleted chromosome 8 likely necessitated the formation of a new kinetochore at the 8p22 neocentromere to stabilize the chromosome during mitosis.

CONCLUSION

This case clearly illustrates the utilization of classic cytogenetics, FISH, and array technologies to better characterize chromosomal abnormalities and provide information on recurrence risks. It also represents a rare case where a neocentromere can form even in the presence of existing alpha-satellite DNA.

Tetrameric organization of vertebrate centromeric nucleosomes

Mitosis ensures equal genome segregation in the eukaryotic lineage. This process is facilitated by microtubule attachment to each chromosome via its centromere. In centromeres, canonical histone H3 is replaced in nucleosomes by a centromere-specific histone H3 variant (CENH3), providing the unique epigenetic signature required for microtubule binding. Due to recent findings of alternative CENH3 nucleosomal forms in invertebrate centromeres, it has been debated whether the classical octameric nucleosomal arrangement of two copies of CENH3, H4, H2A, and H2B forms the basis of the vertebrate centromere. To address this question directly, we examined CENH3 [centromere protein A (CENP-A)] nucleosomal organization in human cells, using a combination of nucleosome component analysis, atomic force microscopy (AFM), and immunoelectron microscopy (immuno-EM). We report that native CENP-A nucleosomes contain centromeric alpha satellite DNA, have equimolar amounts of H2A, H2B, CENP-A, and H4, and bind kinetochore proteins. These nucleosomes, when measured by AFM, yield one-half the dimensions of canonical octameric nucleosomes. Using immuno-EM, we find that one copy of CENP-A, H2A, H2B, and H4 coexist in CENP-A nucleosomes, in which internal C-terminal domains are accessible. Our observations indicate that CENP-A nucleosomes are organized as asymmetric heterotypic tetramers, rather than canonical octamers. Such altered nucleosomes form a chromatin fiber with distinct folding characteristics, which we utilize to discriminate tetramers directly within bulk chromatin. We discuss implications of our observations in the context of universal epigenetic and mechanical requirements for functional centromeres.

Advanced age increases chromosome structural abnormalities in human spermatozoa

This study explores the relationship between sperm structural aberrations and age by using a multicolor multichromosome FISH strategy that provides information on the incidence of duplications and deletions on all the autosomes. ToTelvysion kit (Abbott Molecular, Abbott Park, IL, USA) with telomere-specific probes was used. We investigated the sperm of 10 male donors aged from 23 to 74 years old. The donors were divided into two groups according to age, a cohort of five individuals younger than 40 and a cohort of five individuals older than 60 years. The goal of this study was to determine (1) the relationship between donor age and frequency and type of chromosome structural abnormalities and (2) chromosomes more frequently involved in sperm structural aberrations. We found that the older patients had a higher rate of structural abnormalities (6.6%) compared with the younger cohort (4.9%). Although both duplications and deletions were seen more frequently in older men, our findings demonstrate the presence of an excess of duplications versus deletions in both groups at a ratio of 2 to 1. We demonstrate that the distribution of duplications and deletions was not linear along the chromosomes, although a trend toward a higher rate of abnormalities in larger chromosomes was observed. This work is the first study addressing the frequencies of sperm chromosome structural aberrations of all autosomes in a single assay thus making a contribution to the clarification of the amount and origin of damage present in human spermatozoa and in relation to age.

A case of angioimmunoblastic T-cell non-Hodgkin lymphoma with a neocentric inv dup(1)

Neocentromeres are rare epigenetic phenomena in which functional centromeres are formed onto novel chromosomal locations without any alpha-satellite DNA. To date, constitutional human neocentromeres have been reported in at least 90 cases. In cancer, however, the knowledge is much more limited. Acquired neocentromeres have been described in a particular class of lipomatous tumors (atypical lipomas and well-differentiated liposarcomas; ALP-WDLPS), three cases of acute myeloid leukemia (AML), one case of non-Hodgkin lymphoma (NHL), and one case of lung carcinoma. Here, we report on a 66-year-old male with angioimmunoblastic T-cell NHL. Cytogenetic analysis of his bone marrow showed multiple aberrations, including the presence of a supernumerary chromosome. Using the fluorescence in situ hybridization technique, the supernumerary chromosome was demonstrated to be entirely composed of material derived from chromosome 1. It represented an inverted duplication of the segments between 1q21 and 1qter with a neocentromere in band 1q31. To our knowledge, this is the second reported case of NHL (both T-cell) with the presence of a neocentromere. The occurrence of neocentromeres in tumor cells, however, may be underestimated because of technical limitations during the routine diagnostic chromosomal analysis. The prognostic impact is therefore currently unknown.

Epigenomics of centromere assembly and function

The centromere is a complex chromosomal locus where the kinetochore is formed and microtubules attach during cell division. Centromere identity involves both genomic and sequence-independent (epigenetic) mechanisms. Current models for how centromeres are formed and, conversely, turned off have emerged from studies of unusual or engineered chromosomes, such as neocentromeres, artificial chromosomes, and dicentric chromosomes. Recent studies have highlighted the importance of unique chromatin marked by the histone H3 variant CENP-A, classical chromatin (heterochromatin and euchromatin), and transcription during centromere activation and inactivation. These advances have deepened our view of what defines a centromere and how it behaves in various genomic and chromatin contexts.

Centromere identity: a challenge to be faced

The centromere is a genetic locus, required for faithful chromosome segregation, where spindle fibers attach to the chromosome through kinetochore. Loss of centromere or formation of multiple centromeres on a single chromosome leads to chromosome missegregation or chromosome breakage, respectively, which are detrimental for fitness and survival of a cell. Therefore, understanding the mechanism of centromere locus determination on the chromosome and perpetuation of such a locus in subsequent generation (known as centromere identity) is very fundamental to combat conditions like aneuploidy, spontaneous abortion, developmental defects, cell lethality and cancer. Recent studies have come up with different models to explain centromere identity. However, the exact mechanism still remains elusive. It has been observed that most eukaryotic centromeres are determined epigenetically rather than by a DNA sequence. The epigenetic marks that are instrumental in determining centromere identity are the histone H3 variant, CENP-A and the specialized posttranslational modification of the core histones. Here we will review the recent studies on the factors responsible for generating unique centromeric chromatin and how it perpetuates during cell division giving the present-day models. We will further focus on the probable mechanism of de novo centromere formation with an example of neocentromere. As a matter of similitude, this review will include marking extrachromosomal chromatin to be served as a partitioning locus by deposition of CENP-A homolog in budding yeast.

A paucity of heterochromatin at functional human neocentromeres

Background

Centromeres are responsible for the proper segregation of replicated chromatids during cell division. Neocentromeres are fully functional ectopic human centromeres that form on low-copy DNA sequences and permit analysis of centromere structure in relation to the underlying DNA sequence. Such structural analysis is not possible at endogenous centromeres because of the large amounts of repetitive alpha satellite DNA present.

Results

High-resolution chromatin immunoprecipitation (ChIP) on CHIP (microarray) analysis of three independent neocentromeres from chromosome 13q revealed that each neocentromere contained ~100 kb of centromere protein (CENP)-A in a two-domain organization. Additional CENP-A domains were observed in the vicinity of neocentromeres, coinciding with CpG islands at the 5' end of genes. Analysis of histone H3 dimethylated at lysine 4 (H3K4me2) revealed small domains at each neocentromere. However, these domains of H3K4me2 were also found in the equivalent non-neocentric chromosomes. A surprisingly minimal (~15 kb) heterochromatin domain was observed at one of the neocentromeres, which formed in an unusual transposon-free region distal to the CENP-A domains. Another neocentromere showed a distinct absence of nearby significant domains of heterochromatin. A subtle defect in centromere cohesion detected at these neocentromeres may be due to the paucity of heterochromatin domains.

Conclusions

This high-resolution mapping suggests that H3K4me2 does not seem sufficiently abundant to play a structural role at neocentromeres, as proposed for endogenous centromeres. Large domains of heterochromatin also do not appear necessary for centromere function. Thus, this study provides important insight into the structural requirements of human centromere function.

Centromeres: assembling and propagating epigenetic function

The faithful replication of DNA and the accurate segregation of genomic material from one generation to the next is critical in the maintenance of genomic stability. This chapter will describe the structure and assembly of an epigenetically inherited locus, the centromere, and its role in the processes by which sister chromatids are evenly segregated to daughter cells. During the G2 phase of the cell cycle kinetochores are assembled upon the chromatids. During mitosis, kinetochores attach chromosome(s) to the mitotic spindle. The kinetochore structure serves as the interface between the mitotic spindle and the chromatids and it is at the kinetochore where the forces that drive chromatid separation are generated. Unattached chromosomes fail to satisfy the spindle assembly checkpoint (SAC), resulting in cell cycle arrest. The centromere is the locus upon which the kinetochore assembles, and centromeres themselves are determined by their unique protein composition. Apart from budding yeast, centromeres are not specified simply by DNA sequence, but rather through chromatin composition and architecture and are thus epigenetically determined. Centromeres are built on a specific nucleosome not found elsewhere in the genome, in which histone H3 is replaced with a homologue - CENP-A or CenH3. This domain is flanked by heterochromatin and is folded to provide a 3-dimensional cylinder-like structure at metaphase that establishes the kinetochore on the surface of the mitotic chromosomes. A large family of CENtromere Proteins (CENPs) associates with centromeric chromatin throughout the cell cycle and are required for kinetochore function. Unlike the bulk of histones, CENP-A is not assembled concurrently with DNA synthesis in S-phase but rather assembles into the centromere in the subsequent G1 phase. The assembly of CENP-A chromatin following DNA replication and the re-establishment of this network of constitutive proteins have emerged as critical mechanisms for understanding how the centromere is replicated during the cell cycle.

A case of childhood acute myeloid leukemia AML (M5) with a neocentric chromosome neo(1)(qter-->q23 approximately 24::q23 approximately 24-->q43-->neo-->q43-->qter) and tetrasomy of chromosomes 8 and 21

Hyperdiploidy is rarely observed in childhood acute myeloid leukemia (AML). Described here is the case of a 2(1/2)-year-old girl with AML-M5 and 51 chromosomes characterized by double tetrasomy of chromosomes 8 and 21 and also a neocentric derivative chromosome neo(1)(qter-->q23 approximately 24::q23 approximately 24-->q43-->neo-->q43-->qter). Little is known about the prognostic significance of these chromosomal abnormalities in childhood AML. In the actual case, complete remission was achieved after chemotherapy, which continued for 7 months. No acquired neocentric chromosome 1 has been described previously, even though neocentromere formation has been reported for other chromosomes in neoplasms.

Focus on the centre: the role of chromatin on the regulation of centromere identity and function

The centromere is a specialised chromosomal structure that regulates faithful chromosome segregation during cell division, as it dictates the site of assembly of the kinetochore, a critical structure that mediates binding of chromosomes to the spindle, monitors bipolar attachment and pulls chromosomes to the poles during anaphase. Identified more than a century ago as the primary constriction of condensed metaphase chromosomes, the centromere remained elusive to molecular characterisation for many years owed to its unusual enrichment in highly repetitive satellite DNA sequences, except in budding yeast. In the last decade, our understanding of centromere structure, organisation and function has increased tremendously. Nowadays, we know that centromere identity is determined epigenetically by the formation of a unique type of chromatin, which is characterised by the presence of the centromere-specific histone H3 variant CenH3, originally called CENP-A, which replaces canonical histone H3 at centromeres. CenH3-chromatin constitutes the physical and functional foundation for kinetochore assembly. This review explores recent studies addressing the structural and functional characterisation of CenH3-chromatin, its assembly and propagation during mitosis, and its contribution to kinetochore assembly.

At the right place at the right time: novel CENP-A binding proteins shed light on centromere assembly

Centromeres, the chromosomal loci that form the sites of attachment for spindle microtubules during mitosis, are identified by a unique chromatin structure generated by nucleosomes containing the histone H3 variant CENP-A. The apparent epigenetic mode of centromere inheritance across mitotic and meiotic divisions has generated much interest in how CENP-A assembly occurs and how structurally divergent centromeric nucleosomes can specify the centromere complex. Although a substantial number of proteins have been implicated in centromere assembly, factors that can bind CENP-A specifically and deliver nascent protein to the centromere were, thus far, lacking. Several recent reports on experiments in fission yeast and human cells have now shown significant progress on this problem. Here, we discuss these new developments and their implications for epigenetic centromere inheritance.

Neocentromeres form efficiently at multiple possible loci in Candida albicans

Centromeres are critically important for chromosome stability and integrity. Most eukaryotes have regional centromeres that include long tracts of repetitive DNA packaged into pericentric heterochromatin. Neocentromeres, new sites of functional kinetochore assembly, can form at ectopic loci because no DNA sequence is strictly required for assembly of a functional kinetochore. In humans, neocentromeres often arise in cells with gross chromosome rearrangements that rescue an acentric chromosome. Here, we studied the properties of centromeres in Candida albicans, the most prevalent fungal pathogen of humans, which has small regional centromeres that lack pericentric heterochromatin. We functionally delimited centromere DNA on Chromosome 5 (CEN5) and then replaced the entire region with the counter-selectable URA3 gene or other marker genes. All of the resulting cen5Δ::URA3 transformants stably retained both copies of Chr5, indicating that a functional neocentromere had assembled efficiently on the homolog lacking CEN5 DNA. Strains selected to maintain only the cen5Δ::URA3 homolog and no wild-type Chr5 homolog also grew well, indicating that neocentromere function is independent of the presence of any wild-type CEN5 DNA. Two classes of neocentromere (neoCEN) strains were distinguishable: “proximal neoCEN” and “distal neoCEN” strains. Neocentromeres in the distal neoCEN strains formed at loci about 200–450 kb from cen5Δ::URA3 on either chromosome arm, as detected by massively parallel sequencing of DNA isolated by CENP-A^Cse4p chromatin immunoprecipitation (ChIP). In the proximal neoCEN strains, the neocentromeres formed directly adjacent to cen5Δ::URA3 and moved onto the URA3 DNA, resulting in silencing of its expression. Functional neocentromeres form efficiently at several possible loci that share properties of low gene density and flanking repeated DNA sequences. Subsequently, neocentromeres can move locally, which can be detected by silencing of an adjacent URA3 gene, or can relocate to entirely different regions of the chromosome. The ability to select for neocentromere formation and movement in C. albicans permits mechanistic analysis of the assembly and maintenance of a regional centromere.

Centromere function is essential for proper chromosomal segregation. Most organisms, including humans, have regional centromeres in which centromere function is not strictly dependent on DNA sequence. Upon alteration of chromosomes, new functional centromeres (neocentromeres) can form at ectopic positions. The mechanisms of neocentromere formation are not understood, primarily because neocentromere formation is rarely detected. Here. we show that C. albicans, an important fungal pathogen of humans, has small regional centromeres and can form neocentromeres very efficiently when normal centromere DNA is deleted, and the resulting chromosomes are stably propagated. Neocentromeres can form either very close to the position of the deleted centromere or at other positions along the chromosome arms, including at the telomeres. Subsequently, neocentromeres can move to new chromosomal positions, and this movement can be detected by silencing of a counterselectable gene. The features common to sites of neocentromere formation are longer-than-average intergenic regions and the proximity of inverted or direct repeat sequences. The ability to select for neocentromere formation and movement in C. albicans permits mechanistic analysis of the assembly and maintenance of a regional centromere.

LINE retrotransposon RNA is an essential structural and functional epigenetic component of a core neocentromeric chromatin

We have previously identified and characterized the phenomenon of ectopic human centromeres, known as neocentromeres. Human neocentromeres form epigenetically at euchromatic chromosomal sites and are structurally and functionally similar to normal human centromeres. Recent studies have indicated that neocentromere formation provides a major mechanism for centromere repositioning, karyotype evolution, and speciation. Using a marker chromosome mardel(10) containing a neocentromere formed at the normal chromosomal 10q25 region, we have previously mapped a 330-kb CENP-A–binding domain and described an increased prevalence of L1 retrotransposons in the underlying DNA sequences of the CENP-A–binding clusters. Here, we investigated the potential role of the L1 retrotransposons in the regulation of neocentromere activity. Determination of the transcriptional activity of a panel of full-length L1s (FL-L1s) across a 6-Mb region spanning the 10q25 neocentromere chromatin identified one of the FL-L1 retrotransposons, designated FL-L1b and residing centrally within the CENP-A–binding clusters, to be transcriptionally active. We demonstrated the direct incorporation of the FL-L1b RNA transcripts into the CENP-A–associated chromatin. RNAi-mediated knockdown of the FL-L1b RNA transcripts led to a reduction in CENP-A binding and an impaired mitotic function of the 10q25 neocentromere. These results indicate that LINE retrotransposon RNA is a previously undescribed essential structural and functional component of the neocentromeric chromatin and that retrotransposable elements may serve as a critical epigenetic determinant in the chromatin remodelling events leading to neocentromere formation.

The centromere is an essential chromosomal structure for the correct segregation of chromosomes during cell division. Normal human centromeres comprise a 171-bp α-satellite DNA arranged into tandem and higher-order arrays. Neocentromeres are fully functional centromeres that form epigenetically on noncentromeric regions of the chromosomes, with recent evidence indicating an important role they play in centromere repositioning, karyotype evolution, and speciation. Neocentromeres contain fully definable DNA sequences and provide a tractable system for the molecular analysis of the centromere chromatin. Here, the authors investigate the role of epigenetic determinants in the regulation of neocentromere structure and function. They identify that a retrotransposable DNA element found within the neocentromere domain is actively transcribed and that the transcribed RNA is essential for the structural and functional integrity of the neocentromere. This study defines a previously undescribed epigenetic determinant that regulates the neocentromeric chromatin and provides insight into the mechanism of neocentromere formation and centromere repositioning.

Evolutionary new centromeres in primates

The centromere has a pivotal role in structuring chromosomal architecture, but remains a poorly understood and seemingly paradoxical "black hole." Centromeres are a very rapidly evolving segment of the genome and it is now known that centromere shifts in evolution are not rare and must be considered on a par with other chromosome rearrangements. Recently, unprecedented findings on neocentromeres and evolutionary new centromeres (ENC) have helped clarify the relationship of the centromere within the genome and shown that these two phenomena are two faces of the same coin. No prominent sequence features are known that promote centromere formation and both types of new centromeres are formed epigenetically, both clinical neocentromeres and ENC cluster at chromosomal "hotspots." The clustering of neocentromeres in 8p is probably the result of the relatively high frequency of noncanonical pairing. Studies on the evolution of the chromosomes 3, 13, and 15 help explain why there are clusters of neocentromeres. These domains often correspond to ancestral inactivated centromeres and some regions can preserve features that trigger neocentromere emergence over tens of millions of years. Neocentromeres may be correlated with the distribution of segmental duplications (SDs) in regions of extreme plasticity that often can be characterized as gene deserts. Further, because centromeres and associated pericentric regions are dynamically complex, centromere shifts may turbocharge genome reorganization by influencing the distribution of heterochromatin. The "reuse" of regions as centromere seeding-points in evolution and in human clinical cases further extends the concept of "reuse" of specific domains for "chromosomal events."

The role of ncRNA in centromeres: a lesson from marsupials

Though centromeres have been thought to be comprised of repetitive, transcriptionally inactive DNA, new evidence suggests that eukaryotic centromeres produce a variety of transcripts and that RNA is essential for centromere competence. It has been proposed that centromere satellite transcripts play an essential role in centromere function through demarcation of the kinetochore-binding domain. However, the regional limits and regulation of transcription within the mammalian centromere are unknown. Analysis of transcriptional domains within the centromere in mammalian models is impeded by the unbridgeable expanse of satellite monomers throughout the pericentromere. The comparatively small size of the wallaby centromere and the evolutionary role of the centromere in marsupial speciation events position the wallaby centromere as a tractable and valuable mammalian centromere model. We highlight the current understanding of the wallaby centromere and the role of transcription in centromere function.

Epigenetic regulation of centromeric chromatin: old dogs, new tricks?

The assembly of just a single kinetochore at the centromere of each sister chromatid is essential for accurate chromosome segregation during cell division. Surprisingly, despite their vital function, centromeres show considerable plasticity with respect to their chromosomal locations and activity. The establishment and maintenance of centromeric chromatin, and therefore the location of kinetochores, is epigenetically regulated. The histone H3 variant CENP-A is the key determinant of centromere identity and kinetochore assembly. Recent studies have identified many factors that affect CENP-A localization, but their precise roles in this process are unknown. We build on these advances and on new information about the timing of CENP-A assembly during the cell cycle to propose new models for how centromeric chromatin is established and propagated.

Centromeres in cell division, evolution, nuclear organization and disease

As the spindle fiber attachment region of the chromosome, the centromere has been investigated in a variety of contexts. Here, we will review current knowledge about this unique chromosomal region and its relevance for proper cell division, speciation, and disease. Understanding the three-dimensional organization of centromeres in normal and tumor cells is just beginning to emerge. Multidisciplinary research will allow for new insights into its normal and aberrant nuclear organization and may allow for new therapeutic interventions that target events linked to centromere function and cell division.

Variability of origin for the neocentromeric sequences in analphoid supernumerary marker chromosomes of well-differentiated liposarcomas

Well-differentiated liposarcomas (WDLPS) and dedifferentiated liposarcomas are cytogenetically characterized by the presence of supernumerary ring or giant chromosomes containing amplified material from the 12q14-15 region. These chromosomes contain neocentromeres, which are able to bind the kinetochore proteins and to ensure a stable mitotic transmission although they do not show detectable alpha-satellite sequences. WDLPS is the sole solid tumor for which the presence of a neocentromere is a consistent and specific feature. By immunostaining with anti-centromere antibodies in combination with FISH analysis (immunoFISH) in four cases of WDLPS, we have shown that sequences from the region 12q14-21 region were not located at the neocentromere site. In addition, we have microdissected the neocentromeric region from a giant supernumerary chromosome in the 94T778 WDLPS cell line. By using immunoFISH and positional cloning we have shown that the neocentromere of this cell line originated from a region at 4p16.1, rich in AT sequences and in long interspersed nucleotide element (LINE)1, that was co-amplified with 12q14-15. We have observed that this 4p sequence was not involved in the neocentromere of the supernumerary giant chromosome present in the 93T449 WDLPS cell line derived from a metachronous recurrence of the same primary WDLPS than 94T778. Altogether, these results indicate that the neocentromeres in WDLPS originate from amplified chromosomal regions other than 12q14-15 and do not involve a specific and recurrent DNA sequence. These sequences might be activated for centromeric function by epigenetic mechanisms.

A high-resolution radiation hybrid map of rhesus macaque chromosome 5 identifies rearrangements in the genome assembly

A 10,000-rad radiation hybrid (RH) cell panel of the rhesus macaque was generated to construct a comprehensive RH map of chromosome 5. The map represents 218 markers typed in 185 RH clones. The 4846-cR map has an average marker spacing of 798 kb. Alignments of the RH map to macaque and human genome sequences confirm a large inversion and reveal a previously unreported telomeric inversion. The macaque genome sequence indicates small translocations from the ancestral homolog of macaque chromosome 5 to macaque chromosomes 1 and 6. The RH map suggests that these are probably assembly artifacts. Unlike the genome sequence, the RH mapping data indicate the conservation of synteny between macaque chromosome 5 and human chromosome 4. This study shows that the 10,000-rad panel is appropriate for the generation of a high-resolution whole-genome RH map suitable for the verification of the rhesus genome assembly.

Phylogenomics of African guenons

The karyotypes of 28 specimens belonging to 26 species of Cercopithecinae have been compared with each other and with human karyotype by chromosome banding and, for some of them, by Zoo-FISH (human painting probes) techniques. The study includes the first description of the karyotypes of four species and a synonym of Cercopithecus nictitans. The chromosomal homologies obtained provide us with new data on a large number of rearrangements. This allows us to code chromosomal characters to draw Cercopithecini phylogenetic trees, which are compared to phylogenetic data based on DNA sequences. Our findings show that some of the superspecies proposed by Kingdon (1997 The Kingdon Field Guide to African Mammals, Academic Press.) and Groves (2001 Primates Taxonomy, Smithsonian Institution Press) do not form homogeneous groups and that the genus Cercopithecus is paraphyletic, in agreement with previous molecular analyses. The evolution of Cercopithecini karyotypes is mainly due to non-centromeric chromosome fissions and centromeric shifts or inversions. Non-Robertsonian translocations occurred in C. hamlyni and C. neglectus. The position of chromosomal rearrangements in the phylogenetic tree leads us to propose that the Cercopithecini evolution proceeded by either repeated fission events facilitated by peculiar genomic structures or successive reticulate phases, in which heterozygous populations for few rearranged chromosomes were present, allowing the spreading of chromosomal forms in various combinations, before the speciation process.

Epigenetic control of centromere behavior

The centromere is the DNA region that ensures genetic stability and is therefore of vital importance. Paradoxically, centromere proteins and centromeric structural domains are conserved despite that fact that centromere DNA sequences are highly variable and are not conserved. Remarkably, heritable states at the centromere can be propagated independent of the underlying centromeric DNA sequences. This review describes the epigenetic mechanisms governing centromere behavior, i.e., the mechanisms that control centromere assembly and propagation. A centromeric histone variant, CenH3, and histone modifications play key roles at centromeric chromatin. Histone modifications and RNA interference are important in assembly of pericentric heterochromatin structures. The molecular machinery that is directly involved in epigenetic control of centromeres is shared with regulation of gene expression. Nucleosome remodeling factors, histone chaperones, histone-modifying enzymes, transcription factors, and even RNA polymerase II itself control epigenetic states at centromeres.

Functional analysis of the Arabidopsis centromere by T-DNA insertion-induced centromere breakage

Two minichromosomes (alpha and delta) in addition to two other aberrant chromosomes (beta and gamma) were found in a transgenic Arabidopsis plant produced by an in planta vacuum infiltration technique. The minichromosomes were successfully separated by successive crossing and selfing and added to wild-type Columbia (Col-0) as a supernumerary chromosome. FISH indicated that both of the two minichromosomes originated from the short arm of chromosome 2. The mini alpha chromosome contained the whole short-arm 2S and a truncated centromere (180-bp repeat cluster), whereas mini delta lacked the terminal region including telomere repeats. Pachytene FISH clearly revealed that mini delta comprised a ring chromosome carrying two copies of the region from the 180-bp repeat cluster to BAC-F3C11. Both of the 180-bp clusters (each approximately 500 kb in length) were thought to possess normal centromere functions because the centromere-specific histone H3 variant (HTR12) was detected on both clusters. Notwithstanding this dicentric and ring form, mini delta was stably transmitted to the next generations, perhaps because of its compact size (<4 Mb). Chromosome beta also comprised a dicentric-like structure, with one of the two 180-bp repeat sites derived from chromosome 1 and the other from chromosome 2. However, the latter was quite small and failed to bind HTR12. The data obtained in this study indicated that 500 kb of the 180-bp array of the chromosome 2 centromere, from the edge of the 180-bp array on the short-arm side, is sufficient to form a functional domain.

The mouse A/HeJ Y chromosome: another good Y gone bad

In both humans and mice there are numerous reports of Y chromosome abnormalities that interfere with sex determination. Recent studies in the mouse of one such mutation have identified Y chromosome nondisjunction during preimplantation development as the cause of abnormal testis determination that results in a high frequency of true hermaphroditism. We report here that the mouse Y chromosome from the A/HeJ inbred strain induces similar aberrations in sex determination. Our analyses provide evidence, however, that the mechanism underlying these aberrations is not Y chromosome nondisjunction. On the basis of our findings, we postulate that a mutation at or near the centromere affects both the segregation and sex-determining properties of the A/HeJ Y chromosome. This Y chromosome adds to the growing list of Y chromosome aberrations in humans and mice. In both species, the centromere of the Y is structurally and morphologically distinct from the centromeres of all other chromosomes. We conclude that these centromeric features make the human and mouse Y chromosomes extremely sensitive to minor structural alterations, and that our studies provide yet another example of a good Y chromosome gone 'bad.'

Unexpected structural complexity of supernumerary marker chromosomes characterized by microarray comparative genomic hybridization

BACKGROUND

Supernumerary marker chromosomes (SMCs) are structurally abnormal extra chromosomes that cannot be unambiguously identified by conventional banding techniques. In the past, SMCs have been characterized using a variety of different molecular cytogenetic techniques. Although these techniques can sometimes identify the chromosome of origin of SMCs, they are cumbersome to perform and are not available in many clinical cytogenetic laboratories. Furthermore, they cannot precisely determine the region or breakpoints of the chromosome(s) involved. In this study, we describe four patients who possess one or more SMCs (a total of eight SMCs in all four patients) that were characterized by microarray comparative genomic hybridization (array CGH).

RESULTS

In at least one SMC from all four patients, array CGH uncovered unexpected complexity, in the form of complex rearrangements, that could have gone undetected using other molecular cytogenetic techniques. Although array CGH accurately defined the chromosome content of all but two minute SMCs, fluorescence in situ hybridization was necessary to determine the structure of the markers.

CONCLUSION

The increasing use of array CGH in clinical cytogenetic laboratories will provide an efficient method for more comprehensive characterization of SMCs. Improved SMC characterization, facilitated by array CGH, will allow for more accurate SMC/phenotype correlation.

Species-specific shifts in centromere sequence composition are coincident with breakpoint reuse in karyotypically divergent lineages

The evolution of three classes of centromere sequences across nine species of macropodine marsupials were compared with that of other genes, showing that each species has experienced differential expansion and contraction of individual classes.

Background

It has been hypothesized that rapid divergence in centromere sequences accompanies rapid karyotypic change during speciation. However, the reuse of breakpoints coincident with centromeres in the evolution of divergent karyotypes poses a potential paradox. In distantly related species where the same centromere breakpoints are used in the independent derivation of karyotypes, centromere-specific sequences may undergo convergent evolution rather than rapid sequence divergence. To determine whether centromere sequence composition follows the phylogenetic history of species evolution or patterns of convergent breakpoint reuse through chromosome evolution, we examined the phylogenetic trajectory of centromere sequences within a group of karyotypically diverse mammals, macropodine marsupials (wallabies, wallaroos and kangaroos).

Results

The evolution of three classes of centromere sequences across nine species within the genus Macropus (including Wallabia) were compared with the phylogenetic history of a mitochondrial gene, Cytochrome b (Cyt b), a nuclear gene, selenocysteine tRNA (TRSP), and the chromosomal histories of the syntenic blocks that define the different karyotype arrangements. Convergent contraction or expansion of predominant satellites is found to accompany specific karyotype rearrangements. The phylogenetic history of these centromere sequences includes the convergence of centromere composition in divergent species through convergent breakpoint reuse between syntenic blocks.

Conclusion

These data support the 'library hypothesis' of centromere evolution within this genus as each species possesses all three satellites yet each species has experienced differential expansion and contraction of individual classes. Thus, we have identified a correlation between the evolution of centromere satellite sequences, the reuse of syntenic breakpoints, and karyotype convergence in the context of a gene-based phylogeny.

Evolutionary and clinical neocentromeres: two faces of the same coin?

It has been hypothesized that human clinical neocentromeres and evolutionary novel centromeres (ENC) represent two faces of the same phenomenon. However, there are only two reports of loci harboring both a novel centromere and a clinical neocentromere. We suggest that only the tip of the iceberg has been scratched because most neocentromerization events have a very low chance of being observed. In support of this view, we report here on a neocentromere at 9q33.1 that emerged in a ring chromosome of about 12 Mb. The ring was produced by a balanced rearrangement that was fortuitously discovered because of its malsegregation in the propositus. Chromatin-immunoprecipitation-on-chip experiments using anti-centromere protein (CENP)-A and anti-CENP-C antibodies strongly indicated that a novel centromeric domain was present in the ring, in a chromosomal domain where an ENC emerged in the ancestor to Old World monkeys.

Neocentromeres: new insights into centromere structure, disease development, and karyotype evolution

Since the discovery of the first human neocentromere in 1993, these spontaneous, ectopic centromeres have been shown to be an astonishing example of epigenetic change within the genome. Recent research has focused on the role of neocentromeres in evolution and speciation, as well as in disease development and the understanding of the organization and epigenetic maintenance of the centromere. Here, we review recent progress in these areas of research and the significant insights gained.

Mechanisms and consequences of small supernumerary marker chromosomes: from Barbara McClintock to modern genetic-counseling issues

Supernumerary marker chromosomes (SMCs) are common, but their molecular content and mechanism of origin are often not precisely characterized. We analyzed all centromere regions to identify the junction between the unique chromosome arm and the pericentromeric repeats. A molecular-ruler clone panel for each chromosome arm was developed and used for the design of a custom oligonucleotide array. Of 27 nonsatellited SMCs analyzed by array comparative genomic hybridization (aCGH) and/or fluorescence in situ hybridization (FISH), seven (approximately 26%) were shown to be unique sequence negative. Of the 20 unique-sequence-positive SMCs, the average unique DNA content was approximately 6.5 Mb (range 0.3-22.2 Mb) and 33 known genes (range 0-149). Of the 14 informative nonacrocentric SMCs, five (approximately 36%) contained unique DNA from both the p and q arms, whereas nine (approximately 64%) contained unique DNA from only one arm. The latter cases are consistent with ring-chromosome formation by centromere misdivision, as first described by McClintock in maize. In one case, a r(4) containing approximately 4.4 Mb of unique DNA from 4p was also present in the proband's mother. However, FISH revealed a cryptic deletion in one chromosome 4 and reduced alpha satellite in the del(4) and r(4), indicating that the mother was a balanced ring and deletion carrier. Our data, and recent reports in the literature, suggest that this "McClintock mechanism" of small-ring formation might be the predominant mechanism of origin. Comprehensive analysis of SMCs by aCGH and FISH can distinguish unique-negative from unique-positive cases, determine the precise gene content, and provide information on mechanism of origin, inheritance, and recurrence risk.

Neocentric small supernumerary marker chromosomes (sSMC)--three more cases and review of the literature

Here we report on three new patients with neocentric small supernumerary marker chromosomes (sSMC) derived from chromosome 2, 13 and 15, respectively. The sSMC(13) and sSMC(15) had inverted duplicated shapes and the sSMC(2) a ring chromosome shape. All three cases were clinically severely abnormal. A review of the available sSMC literature revealed that up to the present 73 neocentric sSMC cases including these three new cases have been reported. Seven of these cases were not characterized morphologically; in the remainder, 80% had an inverted duplication, 17% a ring and 3% a minute shape. 81% of the reported neocentric sSMC carriers showed severe, 12% moderate and 8% no clinical abnormalities. In summary, we report three more neocentric sSMC cases, provide a review on all up to now published cases, highlight their special characteristics and compare them to centric sSMC.

Structure, dynamics, and evolution of centromeric nucleosomes

Centromeres are defining features of eukaryotic chromosomes, providing sites of attachment for segregation during mitosis and meiosis. The fundamental unit of centromere structure is the centromeric nucleosome, which differs from the conventional nucleosome by the presence of a centromere-specific histone variant (CenH3) in place of canonical H3. We have shown that the CenH3 nucleosome core found in interphase Drosophila cells is a heterotypic tetramer, a "hemisome" consisting of one molecule each of CenH3, H4, H2A, and H2B, rather than the octamer of canonical histones that is found in bulk nucleosomes. The surprising discovery of hemisomes at centromeres calls for a reevaluation of evidence that has long been interpreted in terms of a more conventional nucleosome. We describe how the hemisome structure of centromeric nucleosomes can account for enigmatic properties of centromeres, including kinetochore accessibility, epigenetic inheritance, rapid turnover of misincorporated CenH3, and transcriptional quiescence of pericentric heterochromatin. Structural differences mediated by loop 1 are proposed to account for the formation of stable tetramers containing CenH3 rather than stable octamers containing H3. Asymmetric CenH3 hemisomes might interrupt the global condensation of octameric H3 arrays and present an asymmetric surface for kinetochore formation. We suggest that this simple mechanism for differentiation between centromeric and packaging nucleosomes evolved from an archaea-like ancestor at the dawn of eukaryotic evolution.

Identification of a “cryptic mosaicism” involving at least four different small supernumerary marker chromosomes derived from chromosome 9 in a woman without reproductive success

OBJECTIVE

To characterize the small supernumerary marker chromosomes (sSMCs) present in the female member of an infertile couple who has no further clinical symptoms.

DESIGN

Case report.

SETTING(S)

Faculty of medicine and institute of human genetics and anthropology.

PATIENT(S)

A young, healthy, nonconsanguineous couple asked for genetic evaluation for infertility.

INTERVENTION(S)

Intracytoplasmic sperm injection, conventional and molecular cytogenetic analyses.

MAIN OUTCOME MEASURE(S)

We characterized the sSMCs present in a woman, who was a member of an infertile couple, by molecular cytogenetic techniques.

RESULT(S)

The G-banding technique showed that a marker chromosome was present in some of the examined cells describing the 47,XX,+mar[30]/46,XX[70] karyotype. Subsequently, using new fluorescence in situ hybridization (FISH) techniques, four distinguishable sSMCs (cryptic mosaicism), all derived from chromosome 9, were observed, including minute and ring chromosomes. This heterogeneity was impossible to detect by the conventional G-banding technique or conventional FISH technique that were used before the new FISH techniques (subcentromere-specific multicolor-FISH [subcenM-FISH]) and specific probe for the 9q12 band. In each metaphase with sSMCs, only one or two markers were observed. On the basis of the FISH analyses, the patient's karyotype was defined as 47,XX,+min(9)(:p12-->q12:)/47,XX,+min(9)(:p12-->q12::q12-->p12:)/47,XX,+r(9)(::p12-->q12::)/47,XX,+r(9)(::p12-->q12::p12-->q12::)x2/46,XX.

CONCLUSION(S)

The presence of sSMCs derived from chromosome 9 could influence the couple's infertility. The new subcenM-FISH techniques are very useful in the characterization of cryptic mosaicisms of marker chromosomes. Additionally, the hypothesis that the 9p12 chromosomal band is an euchromatic variant region without any phenotypic impact other than possible infertility is supported by this case study since the woman shows a normal phenotype.

Repetitive DNA is associated with centromeric domains in Trypanosoma brucei but not Trypanosoma cruzi

Centromeres in Trypanosoma cruzi and Trypanosoma brucei can be localised to regions between directional gene clusters that contain degenerate retroelements, and in the case of T. brucei, repetitive DNA.

Background

Trypanosomes are parasitic protozoa that diverged early from the main eukaryotic lineage. Their genomes display several unusual characteristics and, despite completion of the trypanosome genome projects, the location of centromeric DNA has not been identified.

Results

We report evidence on the location and nature of centromeric DNA in Trypanosoma cruzi and Trypanosoma brucei. In T. cruzi, we used telomere-associated chromosome fragmentation and found that GC-rich transcriptional 'strand-switch' domains composed predominantly of degenerate retrotranposons are a shared feature of regions that confer mitotic stability. Consistent with this, etoposide-mediated topoisomerase-II cleavage, a biochemical marker for active centromeres, is concentrated at these domains. In the 'megabase-sized' chromosomes of T. brucei, topoisomerase-II activity is also focused at single loci that encompass regions between directional gene clusters that contain transposable elements. Unlike T. cruzi, however, these loci also contain arrays of AT-rich repeats stretching over several kilobases. The sites of topoisomerase-II activity on T. brucei chromosome 1 and T. cruzi chromosome 3 are syntenic, suggesting that centromere location has been conserved for more than 200 million years. The T. brucei intermediate and minichromosomes, which lack housekeeping genes, do not exhibit site-specific accumulation of topoisomerase-II, suggesting that segregation of these atypical chromosomes might involve a centromere-independent mechanism.

Conclusion

The localization of centromeric DNA in trypanosomes fills a major gap in our understanding of genome organization in these important human pathogens. These data are a significant step towards identifying and functionally characterizing other determinants of centromere function and provide a framework for dissecting the mechanisms of chromosome segregation.

A large analphoid invdup(3)(q22.3qter) marker chromosome characterized by array-CGH in a child with malformations, mental retardation, ambiguous genitalia and Blaschko's lines

We report a new case of mosaic chromosome 3-derived marker chromosome, present in fibroblasts but not in lymphocytes, found in a child with malformations, mental retardation and ambiguous genitalia. Cytogenetic and molecular analysis showed that the supernumerary invdup(3)(q22.3qter) chromosome was negative at FISH with alpha satellite probe. The presence of a functional neocentromere was confirmed by immunofluorescence with antibodies to centromere proteins (CENPs). Definition of the marker breakpoints has been done through array-CGH. The skin of the patient presented dyschromic areas ordered along Blaschko's lines. The invdup(3q) marker chromosome was present only in fibroblasts from the dark skin biopsy, while lymphocytes and fibroblasts from the normal skin showed a normal male karyotype. Expression of the HPS3 gene (MIM: 606118) was more than two times higher in dark skin fibroblasts. Neocentromeres are most often observed on chromosomal arm fragments that have separated from an endogenous centromere, and therefore actually confer mitotic stability to what would have been acentric fragments. To our knowledge, this invdup(3q) analphoid marker is the largest among the several reported so far. Parental origin and possible mode of formation have been defined by DNA polymorphisms studies. The size of the duplicated marker chromosome and its frequency and tissue distribution may be relevant to the severity of the propositus' phenotype.

Characterization of a neocentric supernumerary marker chromosome originating from the Xp distal region by FISH, CENP-C staining, and array CGH

A small supernumerary marker chromosome (SMC) was observed in a girl with severe developmental delay. Her dysmorphism included prominent forehead, hypertelorism, down-slanting palpebral fissures, low-set/large ears, and flat nasal bridge with anteverted nares. This case also presented hypotonia, hypermobility of joints, congenital heart defect, umbilical hernia, failure to thrive, and seizures. The SMC originated from the distal region of Xp as identified by FISH with multiple DNA probes. Staining with antibodies to Centromere Protein C (CENP-C) demonstrated a neocentromere, while FISH with an alpha-satellite DNA probe showed no hybridization to the SMC. A karyotype was described as 47,XX,+neo(X)(pter-->p22.31::p22.31-->pter), indicating a partial tetrasomy of Xp22.31-->pter. This karyotype represents a functional trisomy for Xp22.31-->pter and a functional tetrasomy for the pseudoautosomal region given that there is no X-inactivation center in the marker chromosome. The SMC was further characterized by microarray-based comparative genomic hybridization (array CGH) as a duplicated DNA fragment of approximately 13 megabase pairs containing about 100 genes. We have described here a new neocentromere with discussion of its clinical significance.

Consensus higher order repeats and frequency of string distributions in human genome

Key string algorithm (KSA) could be viewed as robust computational generalization of restriction enzyme method. KSA enables robust and effective identification and structural analyzes of any given genomic sequences, like in the case of NCBI assembly for human genome. We have developed a method, using total frequency distribution of all r-bp key strings in dependence on the fragment length l, to determine the exact size of all repeats within the given genomic sequence, both of monomeric and HOR type. Subsequently, for particular fragment lengths equal to each of these repeat sizes we compute the partial frequency distribution of r-bp key strings; the key string with highest frequency is a dominant key string, optimal for segmentation of a given genomic sequence into repeat units. We illustrate how a wide class of 3-bp key strings leads to a key-string-dependent periodic cell which enables a simple identification and consensus length determinations of HORs, or any other highly convergent repeat of monomeric or HOR type, both tandem or dispersed. We illustrated KSA application for HORs in human genome and determined consensus HORs in the Build 35.1 assembly. In the next step we compute suprachromosomal family classification and CENP-B box / pJalpha distributions for HORs. In the case of less convergent repeats, like for example monomeric alpha satellite (20-40% divergence), we searched for optimal compact key string using frequency method and developed a concept of composite key string (GAAAC--CTTTG) or flexible relaxation (28 bp key string) which provides both monomeric alpha satellites as well as alpha monomer segmentation of internal HOR structure. This method is convenient also for study of R-strand (direct) / S-strand (reverse complement) alpha monomer alternations. Using KSA we identified 16 alternating regions of R-strand and S-strand monomers in one contig in choromosome 7. Use of CENP-B box and/or pJalpha motif as key string is suitable both for identification of HORs and monomeric pattern as well as for studies of CENP-B box / pJalpha distribution. As an example of application of KSA to sequences outside of HOR regions we present our finding of a tandem with highly convergent 3434-bp Long monomer in chromosome 5 (divergence less then 0.3%).

Late-appearing pseudocentric fission event during chronic myeloid leukemia progression

Pseudocentric fission is a rare event consisting of the splitting of one functional centromere into two new products, of which only one can give rise to a functionally competent kinetochore. We report here a pseudocentric fission event within the D5Z2 alphoid subset disrupting the centromeric region of chromosome 5 in a case of chronic myeloid leukemia (CML) after treatment with imatinib and interferon. The breakage generated unequal partitioning of alpha-satellite sequences between the two fission products. One product was inserted within the long arm of chromosome 12 at band 14.3, becoming the only functional centromere of chromosome der(5). The other fission product was rearranged to form a sandwich-like dicentric--but functionally monocentric--chromosome der(6), made up of material from chromosomes 5, 12, and 6. The intercentric distance on der(6) was shown to be largely >20 Mb. To our knowledge, this is the first pseudocentric fission event described in CML. Moreover, our results confirm the susceptibility to breakage of the centromeric region of chromosome 5.

A minimal CENP-A core is required for nucleation and maintenance of a functional human centromere

Chromatin clusters containing CENP-A, a histone H3 variant, are found in centromeres of multicellular eukaryotes. This study examines the ability of alpha-satellite (alphoid) DNA arrays in different lengths to nucleate CENP-A chromatin and form functional kinetochores de novo. Kinetochore assembly was followed by measuring human artificial chromosome formation in cultured human cells and by chromatin immunoprecipitation analysis. The results showed that both the length of alphoid DNA arrays and the density of CENP-B boxes had a strong impact on nucleation, spreading and/or maintenance of CENP-A chromatin, and formation of functional kinetochores. These effects are attributed to a change in the dynamic balance between assembly of chromatin containing trimethyl histone H3-K9 and chromatin containing CENP-A/C. The data presented here suggest that a functional minimum core stably maintained on 30-70 kb alphoid DNA arrays represents an epigenetic memory of centromeric chromatin.

How to build a centromere: from centromeric and pericentromeric chromatin to kinetochore assembly

The assembly of the centromere, a specialized region of DNA along with a constitutive protein complex which resides at the primary constriction and is the site of kinetochore formation, has been puzzling biologists for many years. Recent advances in the fields of chromatin, microscopy, and proteomics have shed a new light on this complex and essential process. Here we review recently discovered mechanisms and proteins involved in determining mammalian centromere location and assembly. The centromeric core protein CENP-A, a histone H3 variant, is hypothesized to designate centromere localization by incorporation into centromere-specific nucleosomes and is essential for the formation of a functional kinetochore. It has been found that centromere localization of centromere protein A (CENP-A), and therefore centromere determination, requires proteins involved in histone deacetylation, as well as base excision DNA repair pathways and proteolysis. In addition to the incorporation of CENP-A at the centromere, the formation of heterochromatin through histone methylation and RNA interference is also crucial for centromere formation. The assembly of the centromere and kinetochore is complex and interdependent, involving epigenetics and hierarchical protein-protein interactions.

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

BACKGROUND

Mammalian 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.

RESULTS

We 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.

CONCLUSION

Centromeric 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.

Coexistence of neocentromeric marker 3q and trisomy 3 in two different tissues in a 3-year-old boy with peripheral T-cell lymphoma: support for a gene dosage effect hypothesis

A very small supernumerary de novo marker chromosome was ascertained during cytogenetic diagnosis of a 3 1/2-year-old boy with peripheral T-cell lymphoma, unspecified. The marker, which was C-band and alpha-satellite DNA negative, was identified in the pleural effusion and involved cervical lymph node whereas the involved bone marrow cells had trisomy 3 as a clone. The neocentromere marker was characterized by multiple probes demonstrating an inversion duplication of the distal portion of chromosome 3q involving the BCL6 gene. Whole or partial trisomy 3q represents one of the most recurrent chromosomal abnormalities occurring in T-cell lymphomas, suggesting that the 3q contains a critical region for the pathogenesis of T-cell lymphoma. Our present case showed that the critical region may reside within the neocentromere marker 3q27~q29 in this case in particular and revealed a different mechanism in increasing gene dosage rather than gene disruption. In addition, this type of neocentromere is one most often reported in constitutional cases. Here, we report its presence in cancer for the first time.