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

Abstract

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.

Figure 1

Constitutional Neocentromeres (A) Sites of constitutive neocentromere formation in the human genome. The known locations of neocentromeres are represented by bars aligned against the chromosome ideograms; black bars represent neocentromere formation on class I marker chromosomes, red bars represent neocentromere formation on class II marker chromosomes, green bars represent sites of centromere repositioning, and gray bars represent unknown chromosomal rearrangements. Adapted from Amor and Choo. (B) Neocentromere hotspots on 13q. Sites of neocentromere formation are shown in yellow within the length of the marker chromosomes, with markers grouped by neocentromere formation within cytogenetic bands. All reported neocentromere cases from chromosome 13 are illustrated: a–h are as described by Warburton et al; i, Morrissette et al.; j, Knegt et al.; k–m are cases 1–3, respectively, as described by Li et al.; n, Barwell et al.; o, Amor et al.; and p, Tonnies et al. Additional mapping data of a and j are from Cardone et al. and b, d, and g from Alonso et al. All marker chromosomes are inverted duplications (for the sake of simplicity, the inversion is not illustrated for these chromosomes), with the exception of two ring chromosomes designated “R.”

Figure 2

Chromosome Rearrangements after Chromatid Breaks That Are the Common Cause of Neocentromere Formation in Humans Single, unreplicated chromatids of a homologous chromosome pair are depicted; break points are indicated with arrowheads, and the neocentromeres colored red. The resulting effect on the karyotype is listed underneath each alternative rearrangement.

Figure 3

Possible Mechanisms for the Formation of Inverted Duplicated Neocentric Marker Chromosomes (A) Formation at mitosis. After chromatid breakage, the acentric fragment can segregate in two possible ways. After subsequent replication, the broken ends of the acentric fragment rejoin to create the inverted duplication. Neocentromere formation can occur at this stage or after further rounds of cell division. If the neocentric fragment segregates with its sister chromatid, the result is partial tetrasomy for the duplicated fragment. On the other hand, if the centric fragment of the chromatid segregates with the neocentric fragment, the broken ends of the centric fragment can be stabilized by telomere restitution, and the result is partial trisomy for the duplicated fragment. (B) Formation at meiosis. An acentric inverted duplicated (inv dup) chromosome is formed through anomalous crossing over during meiosis I and segregates with a normal sister chromatid to yield the gametes depicted. After fertilization, cells will be tetrasomic for the duplicated region. The inv dup marker might form a neocentromere during meiosis (as shown) or during subsequent rounds of mitotic division after fertilization.

Figure 4

The Size and Distribution of Mapped Protein Domains at Neocentromeres (A) Innerkinetochore domain organization at seven neocentromeres. The size of the protein-binding domain for each neocentromere is listed. Known protein-coding genes present within these domains are shown (derived from Ensembl release 44). The discontinuous nature of the mardel(10) and BBB neocentromeres is illustrated. Two possibilities of the layout of the BBB neocentromere are provided—see main text for details. The IMS13q CENPC1 and CENPH domains are marked “? ” to represent an uncertainty as to the size of these domains—the authors of this study suggested that the lower efficiency of ChIP with CENPC1 and CENPH antibodies might prevent the full extent of these domains from being detected. (B) Scaffold domains, protein binding domains, and genes present within two mapped neocentromeres. Known protein-coding genes present within these domains are shown (derived from Ensembl release 44), with expression data from cell lines derived from Saffery et al. and Wong et al. Differentially expressed genes denote two protein-coding genes found to be activated after neocentromere formation. (Domain positions are derived from BAC data from Lo et al., Alonso et al., Cardone et al., and Saffery et al. updated against Ensembl release 44 from the Ensembl project.)

Figure 5

Fine Structural Localization of CENPA at a Human Alphoid Centromere and a Neocentromere A 45-nm-thick section through each chromosome is shown. Chromosomes were sorted by flow cytometry, fixed in acetone, and labeled with a mouse monoclonal anti-human CENPA primary antibody (MBL) and a Ultrasmall gold anti-mouse secondary antibody (Aurion) before postfixation, embedding, and sectioning. The scale bar represents 200 nm.