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      The Past, Present, and Future of Human Centromere Genomics

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          Abstract

          The centromere is the chromosomal locus essential for chromosome inheritance and genome stability. Human centromeres are located at repetitive alpha satellite DNA arrays that compose approximately 5% of the genome. Contiguous alpha satellite DNA sequence is absent from the assembled reference genome, limiting current understanding of centromere organization and function. Here, we review the progress in centromere genomics spanning the discovery of the sequence to its molecular characterization and the work done during the Human Genome Project era to elucidate alpha satellite structure and sequence variation. We discuss exciting recent advances in alpha satellite sequence assembly that have provided important insight into the abundance and complex organization of this sequence on human chromosomes. In light of these new findings, we offer perspectives for future studies of human centromere assembly and function.

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          Most cited references86

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          Centromeric chromatin exhibits a histone modification pattern that is distinct from both euchromatin and heterochromatin.

          Post-translational histone modifications regulate epigenetic switching between different chromatin states. Distinct histone modifications, such as acetylation, methylation and phosphorylation, define different functional chromatin domains, and often do so in a combinatorial fashion. The centromere is a unique chromosomal locus that mediates multiple segregation functions, including kinetochore formation, spindle-mediated movements, sister cohesion and a mitotic checkpoint. Centromeric (CEN) chromatin is embedded in heterochromatin and contains blocks of histone H3 nucleosomes interspersed with blocks of CENP-A nucleosomes, the histone H3 variant that provides a structural and functional foundation for the kinetochore. Here, we demonstrate that the spectrum of histone modifications present in human and Drosophila melanogaster CEN chromatin is distinct from that of both euchromatin and flanking heterochromatin. We speculate that this distinct modification pattern contributes to the unique domain organization and three-dimensional structure of centromeric regions, and/or to the epigenetic information that determines centromere identity.
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            Conserved organization of centromeric chromatin in flies and humans.

            Recent studies have highlighted the importance of centromere-specific histone H3-like (CENP-A) proteins in centromere function. We show that Drosophila CID and human CENP-A appear at metaphase as a three-dimensional structure that lacks histone H3. However, blocks of CID/CENP-A and H3 nucleosomes are linearly interspersed on extended chromatin fibers, and CID is close to H3 nucleosomes in polynucleosomal preparations. When CID is depleted by RNAi, it is replaced by H3, demonstrating flexibility of centromeric chromatin organization. Finally, contrary to models proposing that H3 and CID/CENP-A nucleosomes are replicated at different times in S phase, we show that interspersed H3 and CID/CENP-A chromatin are replicated concurrently during S phase in humans and flies. We propose that the unique structural arrangement of CID/CENP-A and H3 nucleosomes presents centromeric chromatin to the poleward face of the condensing mitotic chromosome.
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              Genomic and genetic definition of a functional human centromere.

              The definition of centromeres of human chromosomes requires a complete genomic understanding of these regions. Toward this end, we report integration of physical mapping, genetic, and functional approaches, together with sequencing of selected regions, to define the centromere of the human X chromosome and to explore the evolution of sequences responsible for chromosome segregation. The transitional region between expressed sequences on the short arm of the X and the chromosome-specific alpha satellite array DXZ1 spans about 450 kilobases and is satellite-rich. At the junction between this satellite region and canonical DXZ1 repeats, diverged repeat units provide direct evidence of unequal crossover as the homogenizing force of these arrays. Results from deletion analysis of mitotically stable chromosome rearrangements and from a human artificial chromosome assay demonstrate that DXZ1 DNA is sufficient for centromere function. Evolutionary studies indicate that, while alpha satellite DNA present throughout the pericentromeric region of the X chromosome appears to be a descendant of an ancestral primate centromere, the current functional centromere based on DXZ1 sequences is the product of the much more recent concerted evolution of this satellite DNA.
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                Author and article information

                Contributors
                Journal
                101551097
                39257
                Genes (Basel)
                Genes (Basel)
                Genes
                2073-4425
                10 March 2014
                24 January 2014
                26 March 2014
                : 5
                : 1
                : 33-50
                Affiliations
                [1 ]Department of Molecular Genetics and Microbiology, Duke University Medical Center, Durham, NC 27710, USA
                [2 ]Division of Human Genetics, Duke University, Durham, NC 27710, USA
                Author notes
                [* ]Author to whom correspondence should be addressed; beth.sullivan@ 123456duke.edu ; Tel.: +1-919-684-9038
                Article
                NIHMS562024
                10.3390/genes5010033
                3966626
                24683489
                ecf5ccff-b27c-4dac-beb1-1a965e80b59e
                © 2014 by the authors; licensee MDPI, Basel, Switzerland.

                This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution license ( http://creativecommons.org/licenses/by/3.0/).

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                Categories
                Article

                alpha satellite,higher order repeat,cenp,heterochromatin,human artificial chromosome,dicentric,chromosome truncation,transcription,tet operon

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