The Role of Human Centromeric RNA in Chromosome Stability

Chromosome instability is a hallmark of cancer and is caused by inaccurate segregation of chromosomes. One cellular structure used to avoid this fate is the kinetochore, which binds to the centromere on the chromosome. Human centromeres are poorly understood, since sequencing and analyzing repeated alpha-satellite DNA regions, which can span a few megabases at the centromere, are particularly difficult. However, recent analyses revealed that these regions are actively transcribed and that transcription levels are tightly regulated, unveiling a possible role of RNA at the centromere. In this short review, we focus on the recent discovery of the function of human centromeric RNA in the regulation and structure of the centromere, and discuss the consequences of dysregulation of centromeric RNA in cancer.

Alpha satellite insertions and the evolutionary landscape of centromeres

Human centromeres are composed of alpha satellite DNA hierarchically organized as higher-order repeats and epigenetically specified by CENP-A binding. Current evolutionary models assert that new centromeres are first epigenetically established and subsequently acquire an alphoid array. We identified during routine prenatal aneuploidy diagnosis by FISH a de novo insertion of alpha satellite DNA array (~50-300 kbp) from the centromere of chromosome 18 (D18Z1) into chromosome 15q26 euchromatin. Although bound by CENP-B, this locus did not acquire centromeric functionality as demonstrated by lack of constriction and absence of CENP-A binding. We characterized the rearrangement by FISH and sequencing using Illumina, PacBio, and Nanopore adaptive sampling which revealed that the insertion was associated with a 2.8 kbp deletion and likely occurred in the paternal germline. Notably, the site was located ~10 Mbp distal from the location where a centromere was ancestrally seeded and then became inactive sometime between 20 and 25 million years ago (Mya), in the common ancestor of humans and apes. Long reads spanning either junction showed that the organization of the alphoid insertion followed the 12-mer higher-order repeat structure of the D18Z1 array. Mapping to the CHM13 human genome assembly revealed that the satellite segment transposed from a specific location of chromosome 18 centromere. The rearrangement did not directly disrupt any gene or predicted regulatory element and did not alter the epigenetic status of the surrounding region, consistent with the absence of phenotypic consequences in the carrier. This case demonstrates a likely rare but new class of structural variation that we name ‘alpha satellite insertion’. It also expands our knowledge about the evolutionary life cycle of centromeres, conveying the possibility that alphoid arrays can relocate near vestigial centromeric sites.

The Cytogenomic “Theory of Everything”: Chromohelkosis May Underlie Chromosomal Instability and Mosaicism in Disease and Aging

Mechanisms for somatic chromosomal mosaicism (SCM) and chromosomal instability (CIN) are not completely understood. During molecular karyotyping and bioinformatic analyses of children with neurodevelopmental disorders and congenital malformations (n = 612), we observed colocalization of regular chromosomal imbalances or copy number variations (CNV) with mosaic ones (n = 47 or 7.7%). Analyzing molecular karyotyping data and pathways affected by CNV burdens, we proposed a mechanism for SCM/CIN, which had been designated as “chromohelkosis” (from the Greek words chromosome ulceration/open wound). Briefly, structural chromosomal imbalances are likely to cause local instability (“wreckage”) at the breakpoints, which results either in partial/whole chromosome loss (e.g., aneuploidy) or elongation of duplicated regions. Accordingly, a function for classical/alpha satellite DNA (protection from the wreckage towards the centromere) has been hypothesized. Since SCM and CIN are ubiquitously involved in development, homeostasis and disease (e.g., prenatal development, cancer, brain diseases, aging), we have metaphorically (ironically) designate the system explaining chromohelkosis contribution to SCM/CIN as the cytogenomic “theory of everything”, similar to the homonymous theory in physics inasmuch as it might explain numerous phenomena in chromosome biology. Recognizing possible empirical and theoretical weaknesses of this “theory”, we nevertheless believe that studies of chromohelkosis-like processes are required to understand structural variability and flexibility of the genome.

Evolutionary History of Alpha Satellite DNA Repeats Dispersed within Human Genome Euchromatin

Major human alpha satellite DNA repeats are preferentially assembled within (peri)centromeric regions but are also dispersed within euchromatin in the form of clustered or short single repeat arrays. To study the evolutionary history of single euchromatic human alpha satellite repeats (ARs), we analyzed their orthologous loci across the primate genomes. The continuous insertion of euchromatic ARs throughout the evolutionary history of primates starting with the ancestors of Simiformes (45–60 Ma) and continuing up to the ancestors of Homo is revealed. Once inserted, the euchromatic ARs were stably transmitted to the descendant species, some exhibiting copy number variation, whereas their sequence divergence followed the species phylogeny. Many euchromatic ARs have sequence characteristics of (peri)centromeric alpha repeats suggesting heterochromatin as a source of dispersed euchromatic ARs. The majority of euchromatic ARs are inserted in the vicinity of other repetitive elements such as L1, Alu, and ERV or are embedded within them. Irrespective of the insertion context, each AR insertion seems to be unique and once inserted, ARs do not seem to be subsequently spread to new genomic locations. In spite of association with (retro)transposable elements, there is no indication that such elements play a role in ARs proliferation. The presence of short duplications at most of ARs insertion sites suggests site-directed recombination between homologous motifs in ARs and in the target genomic sequence, probably mediated by extrachromosomal circular DNA, as a mechanism of spreading within euchromatin.

Heat Stress Affects H3K9me3 Level at Human Alpha Satellite DNA Repeats

Satellite DNAs are tandemly repeated sequences preferentially assembled into large arrays within constitutive heterochromatin and their transcription is often activated by stress conditions, particularly by heat stress. Bioinformatic analyses of sequenced genomes however reveal single repeats or short arrays of satellite DNAs dispersed in the vicinity of genes within euchromatin. Here, we analyze transcription of a major human alpha satellite DNA upon heat stress and follow the dynamics of “silent” H3K9me3 and “active” H3K4me2/3 histone marks at dispersed euchromatic and tandemly arranged heterochromatic alpha repeats. The results show H3K9me3 enrichment at alpha repeats upon heat stress, which correlates with the dynamics of alpha satellite DNA transcription activation, while no change in H3K4me2/3 level is detected. Spreading of H3K9me3 up to 1–2 kb from the insertion sites of the euchromatic alpha repeats is detected, revealing the alpha repeats as modulators of local chromatin structure. In addition, expression of genes containing alpha repeats within introns as well as of genes closest to the intergenic alpha repeats is downregulated upon heat stress. Further studies are necessary to reveal the possible contribution of H3K9me3 enriched alpha repeats, in particular those located within introns, to the silencing of their associated genes.

CHARACTERIZATION OF HUMAN CHROMOSOMAL CONSTITUTIVE HETEROCHROMATIN

Heterochromatin of centromeric chromosome regions contains late replicating, largely repetitive DNA. It is suggested that heterochromatin participates in chromosome pairing, crossing-over and in chromosome disjunction control (1,3). Centromeric heterochromatin, a variety of heterochromatin, is a tightly packed form of DNA.Centromeric heterochromatin is a constituent in the formation ofactive centromeres in most higher-order organisms; the domain exists on both mitotic and interphase chromosomes. (4,5,6,8) Centromeric heterochromatin is usually formed on alpha satellite DNA in humans; however, there have been cases where centric heterochromatin and centromeres have formed on originally euchromatin domains lacking alpha satellite DNA; this usually happens as a result of a chromosome breakage event and the formed centromere is called a neocentromere.

Cytogenetic findings in Serbian patients with Klinefelter syndrome

Klinefelter syndrome (KS) describes the phenotype of the most common sex chromosome abnormality in humans (1/600 newborn males). The most widespread karyotype in affected patients is 47,XXY, but various others have been described. The aim of this study was to examine the karyotypes of a group of patients suspected of having Klinefelter's syndrome. Between January 1993 and April 2018 104 adult KS patients were evaluated. Cytogenetic analysis was carried out on metaphases obtained from phytohemagglutinin-stimulated peripheral lymphocytes using a standard procedure. Fluorescence in situ hybridization (FISH) analysis was performed on peripheral blood specimens. Vysis CEP X/Y- alpha satellite DNA probes were used to detect X and Y chromosomes. We identified KS presenting the ?standard? or 47,XXY karyotype in eighty three (80%) patients, while five (5%) KS patients showed the mosaic karyotype 47,XXY/46,XY and three (3%) patients had the mosaic karyotype 47,XXY/46,XX. In six (6%) cases KS patients with the ?standard? karyotype also had autosomal chromosomal abnormalities, while numerical sex chromosome abnormalities, with karyotypes 48,XXYY occurred in two (2%) subjects, 47,XYY in three (3%) and 47,XYY/46,XY in two (2%) individuals. Thus, most of our KS patients had the 'standard', 47,XXY karyotype, but some men formed a group of patients with a diversity of other karyotypes. These disparate chromosomal variants may have different physical and mental implications for the general symptomatology of KS. Therefore, it is important to determine the nature of the karyotype of every male with clinical characteristics of KS in very early childhood in order to initiate an adequate, personalized, medical approach.

Repetitive Fragile Sites: Centromere Satellite DNA As a Source of Genome Instability in Human Diseases

Maintenance of an intact genome is essential for cellular and organismal homeostasis. The centromere is a specialized chromosomal locus required for faithful genome inheritance at each round of cell division. Human centromeres are composed of large tandem arrays of repetitive alpha-satellite DNA, which are often sites of aberrant rearrangements that may lead to chromosome fusions and genetic abnormalities. While the centromere has an essential role in chromosome segregation during mitosis, the long and repetitive nature of the highly identical repeats has greatly hindered in-depth genetic studies, and complete annotation of all human centromeres is still lacking. Here, we review our current understanding of human centromere genetics and epigenetics as well as recent investigations into the role of centromere DNA in disease, with a special focus on cancer, aging, and human immunodeficiency–centromeric instability–facial anomalies (ICF) syndrome. We also highlight the causes and consequences of genomic instability at these large repetitive arrays and describe the possible sources of centromere fragility. The novel connection between alpha-satellite DNA instability and human pathological conditions emphasizes the importance of obtaining a truly complete human genome assembly and accelerating our understanding of centromere repeats’ role in physiology and beyond.