Article

Telomeric repeats far from the ends: mechanisms of origin and role in evolution. Cytogenet Genome Res

Dipartimento di Genetica e Microbiologia, Adriano Buzzati-Traverso, Università di Pavia, Pavia, Italy.
Cytogenetic and Genome Research (Impact Factor: 1.91). 02/2008; 122(3-4):219-28. DOI: 10.1159/000167807
Source: PubMed

ABSTRACT In addition to their location at terminal positions, telomeric-like repeats are also present at internal sites of the chromosomes (intrachromosomal or interstitial telomeric sequences, ITSs). According to their sequence organization and genomic location, two different kinds of ITSs can be identified: (1) heterochromatic ITSs (het-ITSs), large (up to hundreds of kb) stretches of telomeric-like DNA localized mainly at centromeres, and (2) short ITSs (s-ITSs), short stretches of telomeric hexamers distributed at internal sites of the chromosomes. Het-ITSs have been only described in some vertebrate species and they probably represent the remnants of evolutionary chromosomal rearrangements. On the contrary, s-ITSs are probably present in all mammalian genomes although they have been described in detail only in some completely sequenced genomes. Sequence database analysis revealed the presence of 83, 79, 244 and 250 such s-ITSs in the human, chimpanzee, mouse and rat genomes, respectively. Analysis of the flanking sequences suggested that s-ITSs were inserted during the repair of DNA double-strand breaks that occurred in the course of evolution. An extensive comparative analysis of the s-ITS loci and their orthologous 'empty' loci confirmed this hypothesis and suggested that the repair event involved the direct action of telomerase. Whereas het-ITSs seem to be intrinsically prone to breakage, the instability of s-ITSs is more controversial. This observation is consistent with the hypothesis that s-ITSs are probably not themselves prone to breakage but represent 'scars' of ancient breakage that may have occurred within fragile regions. This study will review the current knowledge on these two types of ITS, their molecular organization, how they arose during evolution, their implications for chromosomal instability and their potential applications as phylogenetic/forensic markers.

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    • "It has been suggested that from many ITRs, only large blocks of telomeric repeats (spanning several hundred kb) are involved in chromosome breakage, whereas instability of short ITRs is more controversial (Lin and Yan 2008; Ruiz-Herrera et al. 2008). ITRs observed in P. echinatum definitely contain a high number of repeats because FISH performed on condensed chromosomes cannot detect target loci smaller than 10 kb. "
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    • "Additionally, telomeric repeat sequences were also present at nontelomeric sites (interstitial telomeric sequences, ITSs). Interstitial telomeric signals were present in 12 pericentromeric regions, coinciding with CBG bands, in P. lamarum and in pair 4 of Phyllomys sp. 4. The origin of ITs is still debated, but it seems that they may reflect components of the satellite DNA, as described in rodents of the genus Microtus (Rovatsos et al. 2011) or remnants of ancestral chromosome rearrangements, such as inversions and centric or tandem fusions (Meyne et al. 1990; Lee et al. 1993; Fagundes et al. 1997; Svartman and Vianna-Morgante 1998; Pellegrino et al. 1999; Bolzán and Bianchi 2006; Ruiz-Herrera et al. 2008). Only three species of Echimyidae (Proechimys gr. "
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