Identification of a human chromosome-specific interstitial telomere-like sequence (ITS) at 22q11.2 using double-strand PRINS

Interstitial telomeric sequences in vertebrate chromosomes: Origin, function, instability and evolution

By definition, telomeric sequences are located at the very ends or terminal regions of chromosomes. However, several vertebrate species show blocks of (TTAGGG)n repeats present in non-terminal regions of chromosomes, the so-called interstitial telomeric sequences (ITSs), interstitial telomeric repeats or interstitial telomeric bands, which include those intrachromosomal telomeric-like repeats located near (pericentromeric ITSs) or within the centromere (centromeric ITSs) and those telomeric repeats located between the centromere and the telomere (i.e., truly interstitial telomeric sequences) of eukaryotic chromosomes. According with their sequence organization, localization and flanking sequences, ITSs can be classified into four types: 1) short ITSs, 2) subtelomeric ITSs, 3) fusion ITSs, and 4) heterochromatic ITSs. The first three types have been described mainly in the human genome, whereas heterochromatic ITSs have been found in several vertebrate species but not in humans. Several lines of evidence suggest that ITSs play a significant role in genome instability and evolution. This review aims to summarize our current knowledge about the origin, function, instability and evolution of these telomeric-like repeats in vertebrate chromosomes.

No Interstitial Telomeres on Autosomes but Remarkable Amplification of Telomeric Repeats on the W Sex Chromosome in the Sand Lizard (Lacerta agilis)

Telomeres are repeat (TTAGGG) n sequences that form terminal ends of chromosomes and have several functions, such as protecting the coding DNA from erosion at mitosis. Due to chromosomal rearrangements through evolutionary history (e.g., inversions and fusions), telomeric sequences are also found between the centromere and the terminal ends (i.e., at interstitial telomeric sites, ITSs). ITS telomere sequences have been implicated in heritable disease caused by genomic instability of ITS polymorphic variants, both with respect to copy number and sequence. In the sand lizard (Lacerta agilis), we have shown that telomere length is predictive of lifetime fitness in females but not males. To assess whether this sex specific fitness effect could be traced to ITSs differences, we mapped (TTAGGG) n sequences using fluorescence in situ hybridization in fibroblast cells cultured from 4 specimens of known sex. No ITSs could be found on autosomes in either sex. However, females have heterogametic sex chromosomes in sand lizards (ZW, 2n = 38) and the female W chromosome showed degeneration and remarkable (TTAGGG) n amplification, which was absent in the Z chromosomes. This work warrants further research on sex chromosome content, in particular of the degenerate W chromosome, and links to female fitness in sand lizards.

Chromosomal aberrations involving telomeres and interstitial telomeric sequences

Telomeres are specialised nucleoproteic complexes localised at the physical ends of linear eukaryotic chromosomes that maintain their stability and integrity. In vertebrate chromosomes, the DNA component of telomeres is constituted by (TTAGGG)n repeats, which can be localised at the terminal regions of chromosomes (true telomeres) or at intrachromosomal sites (interstitial telomeric sequences or ITSs, located at the centromeric region or between the centromere and the telomere). In the past two decades, the use of molecular cytogenetic techniques has led to a new spectrum of spontaneous and clastogen-induced chromosomal aberrations being identified, involving telomeres and ITSs. Some aberrations involve the chromosome ends and, indirectly, the telomeric repeats located at the terminal regions of chromosomes (true telomeres). A second type of aberrations directly involves the telomeric sequences located at the chromosome ends. Finally, there is a third class of aberrations that specifically involves the ITSs. The aims of this review are to provide a detailed description of these aberrations and to summarise the available data regarding their induction by physical and chemical mutagens.

Polymorphism in a human chromosome-specific interstitial telomere-like sequence at 22q11.2

Interstitial telomeric sequences (ITSs) are common in human. We previously reported the presence of an ITS at 22q11.2 which is in the vicinity of the genomically unstable region involved in 22q11 rearrangements. Recently, we studied the molecular status of the ITS 22q11.2 in the normal population. The amplification of an ITS at 22q11.2 showed different patterns ranging from 1-4 kb, confirming the highly polymorphic nature of this sequence. The linkage analysis of the ITS at 22q11.2 in members of 10 different families demonstrated a strong relation between offspring and parents. In contrast, the study of a DiGeorge case and his 2 parents revealed the presence of a novel allele probably inherited from the father. These results open an avenue for the use of this sequence as an allelic marker, and its implication in 22q11.2-related pathogenesis.

[Detecting human chromosome anomalies with primed in situ labeling (PRINS)]

Numerical chromosome anomaly was one of the most important kinds of human chromosome diseases by inducing pregnancy loss, miscarriage, infant death, congenital malformations and nerve damage. The present study was to establish a rapid, reliable and reasonable multicolor primed in situ labeling (PRINS) protocol for diagnosing numerical anomaly in human chromosome. First, nuclei of cultured lymphocytes and sperms were labeled with the method of PRINS, and then nuclei of cultured lymphocytes, sperms and other specimen were labeled with the method of updated non-ddNTP-blocking multicolor PRINS technique. The labeling effect of different target sequences and the feature of different fluorochromes were evaluated by experiment. Meanwhile, several parameters of PRINS were optimized to obtain more homogeneous and stable labeling effect. At last, the applicative value of PRINS was evaluated by comparing the clinical effect and labeling characteristics between FISH probe and PRINS. In the present study, several chromosomes were simultaneously marked successfully in the same sperm nucleus within 2.5 hours. And the frequency of one-color-labeling reached 99%. The many advantages, compared with FISH, make PRINS become the first choice in diagnosing diseases related to numerical anomaly in human chromosome.

Endings in the middle: current knowledge of interstitial telomeric sequences

Interstitial telomeric sequences (ITSs) consist of tandem repeats of the canonical telomeric repeat and are common in mammals. They are localized at intrachromosomal sites, including those repeats located close to the centromeres and those found at interstitial sites, i.e., between the centromeres and the telomeres. ITSs might originate from ancestral intrachromosomal rearrangements (inversions and fusions), from differential crossing-over or from the repair of double-strand break during evolution. Three classes of ITSs have been described in the human genome, namely, short ITSs, long subtelomeric ITSs and fusion ITSs. The fourth class of ITSs, pericentromeric ITSs, has been found in other species. The function of ITSs can be inferred from the association of heritable diseases with ITS polymorphic variants, both in copy number and sequence. This is one of the most attractive aspects of ITS studies because it leads to new and useful markers for genetic linkage studies, forensic applications, and detection of genetic instability in tumors. Some ITSs also might be hotspots of chromosome breakage, rearrangement and amplification sites, based on the type of clastogens and the nature of ITSs. This study will contribute new knowledge with respect to ITSs' biology and mechanism, prevalence of diseases, risk evaluation and prevention of related diseases, thus facilitates the design of early detection markers for diseases caused by genomic instability.