Asynchronous replication timing of telomeres at opposite arms of mammalian chromosomes

Abstract

Telomeres are defining structural elements of all linear chromosomes, yet information concerning the timing of their replication in higher eukaryotes is surprisingly limited. We developed an approach that allowed a study of telomere replication patterns of specific mammalian chromosomes. In the Indian muntjac (Muntiacus muntjac), replication timing between respective telomeres of homologous chromosomes was highly coordinated, but no such synchrony was evident for p-and q-arm telomeres of the same chromosome. This finding contrasts with the coordinated timing of both ends of each chromosome in yeast. Also in contrast to yeast, where replication of all telomeres is confined to late S phase, we found specific telomeres in Indian muntjac chromosomes that replicated early in S and other telomeres that replicated later. Finally, replication timing of some but not all telomeres was influenced by telomere length. Knowledge of telomere replication timing represents a first step toward understanding the relationship between telomere replication and telomerase action. The approach, which we call replicative detargeting fluorescence in situ hybridization, is widely applicable to different species and genetic loci.DNA replication ͉ Indian muntjac ͉ telomerase L ocated at the ends of all eukaryotic chromosomes, telomeres are essential for ensuring chromosomal integrity and maintaining overall genome stability (1-3). In vertebrates, each chromosome end is composed of a sequence of identical tandem TTAGGG sequences (telomeric repeats), which associate with proteins to form a specialized structure to protect chromosomes from end-to-end fusion, degradation, and inappropriate recombination (1-3). The accurate and timely duplication of the genome is a major task for eukaryotic cells and requires the cooperation of multiple factors to ensure the stability of the genetic information of each cell. Telomeres are essential for the complete replication of eukaryotic chromosomes. Replication of linear DNA results in what is commonly known as the end replication problem (4, 5), in which the DNA replication machinery is not able to completely replicate the very termini of chromosomes. As a result, 50 -200 bp of telomeric DNA are lost during each S phase in cultured human cells. This loss can be prevented by telomerase (hTERT), a cellular reverse transcriptase. The catalytic subunit of telomerase contains an integral RNA molecule (hTR) with the sequence that is used as a template to add telomeric repeats onto the 3Ј end of the telomere. Telomeres in yeast are short (Ϸ250 -350 nucleotides) and maintained at relatively constant length because of the constitutive expression of telomerase. Telomeres in both human and Indian muntjac (Muntiacus muntjac, a barking deer) are much longer, ranging from 4 to 20 kb (6 -8). Heterogeneous telomere lengths have been observed at different chromosome ends and even at homologous telomere ends (9, 10). Most human and Indian muntjac fibroblasts do not express telomerase, and thus their telomeres shorten progres...

Controlled light exposure microscopy reveals dynamic telomere microterritories throughout the cell cycle

“…On the other hand, when telomeres shorten, they acquire a more open (epigenetically active) state, probably to allow access to telomere elongation mechanisms (49). In addition, telomeres have been found in close proximity of splicing factor-rich, transcription permissive SC35 domains (17,18), telomeres replicate throughout S-phase, in contrast with constitutive pericentric heterochromatin that is generally late-replicating (50,51) and recently, transcription of telomeres into telomeric RNA was discovered (52,53). Together, these data point to telomeres being unique structures with an ambiguous character bearing distinct heterochromatic and more active properties.…”

A genomic view of eukaryotic DNA replication

“…The subtelomeric sequences of each chromosome replicated at almost precisely the same time, yet, as a whole, different pairs of chromosome telomeres were not co-ordinated. Recent studies in human cells (Zou et al 2004), however, have observed that the subtelomeric regions for each chromosome replicate independently of each other and are not co-ordinated. Careful examination of the yeast replication timing data suggests that the similar time of replication observed for each chromosome's telomeres could be a result of circularizing the chromosomal co-ordinates to generate a continuous data set for the smoothing algorithms.…”

ATM-like kinases and regulation of telomerase: lessons from yeast and mammals

“…Unlike replication of S. cerevisiae telomeres, which occurs at the very end of S phase, semiconservative DNA replication of human telomeres occurs throughout S phase [66,67]. A small amount of telomeric DNA synthesis occurs in post-S phase human cells that do not express telomerase [68].…”