Chromatin structure in telomere dynamics

The establishment of a specific nucleoprotein structure, the telomere, is required to ensure the protection of chromosome ends from being recognized as DNA damage sites. Telomere shortening below a critical length triggers a DNA damage response that leads to replicative senescence. In normal human somatic cells, characterized by telomere shortening with each cell division, telomere uncapping is a regulated process associated with cell turnover. Nevertheless, telomere dysfunction has also been associated with genomic instability, cell transformation, and cancer. Despite the essential role telomeres play in chromosome protection and in tumorigenesis, our knowledge of the chromatin structure involved in telomere maintenance is still limited. Here we review the recent findings on chromatin modifications associated with the dynamic changes of telomeres from protected to deprotected state and their role in telomere functions.

Telomere Replication: Solving Multiple End Replication Problems

Eukaryotic genomes are highly complex and divided into linear chromosomes that require end protection from unwarranted fusions, recombination, and degradation in order to maintain genomic stability. This is accomplished through the conserved specialized nucleoprotein structure of telomeres. Due to the repetitive nature of telomeric DNA, and the unusual terminal structure, namely a protruding single stranded 3' DNA end, completing telomeric DNA replication in a timely and efficient manner is a challenge. For example, the end replication problem causes a progressive shortening of telomeric DNA at each round of DNA replication, thus telomeres eventually lose their protective capacity. This phenomenon is counteracted by the recruitment and the activation at telomeres of the specialized reverse transcriptase telomerase. Despite the importance of telomerase in providing a mechanism for complete replication of telomeric ends, the majority of telomere replication is in fact carried out by the conventional DNA replication machinery. There is significant evidence demonstrating that progression of replication forks is hampered at chromosomal ends due to telomeric sequences prone to form secondary structures, tightly DNA-bound proteins, and the heterochromatic nature of telomeres. The telomeric loop (t-loop) formed by invasion of the 3'-end into telomeric duplex sequences may also impede the passage of replication fork. Replication fork stalling can lead to fork collapse and DNA breaks, a major cause of genomic instability triggered notably by unwanted repair events. Moreover, at chromosomal ends, unreplicated DNA distal to a stalled fork cannot be rescued by a fork coming from the opposite direction. This highlights the importance of the multiple mechanisms involved in overcoming fork progression obstacles at telomeres. Consequently, numerous factors participate in efficient telomeric DNA duplication by preventing replication fork stalling or promoting the restart of a stalled replication fork at telomeres. In this review, we will discuss difficulties associated with the passage of the replication fork through telomeres in both fission and budding yeasts as well as mammals, highlighting conserved mechanisms implicated in maintaining telomere integrity during replication, thus preserving a stable genome.

Telomeric nucleosomes: forgotten players at chromosome ends

Telomeres are the special nucleoprotein structures that protect chromosome ends from both recombination and degradation. In most organisms, telomeric DNA consists of short sequences repeated in tandem ending in single-stranded G-rich overhangs. In higher eukaryotes, about 80% of telomeric DNA is organized in tightly packed nucleosomes separated by 10-20 bp of linker DNA. Several specific proteins contribute to telomeric structure. At the moment, a satisfactory description of telomere organization is still lacking. Whereas the role played by telomeric proteins in telomere function and regulation has been widely investigated, little is known about the contribution of nucleosomes to the protection of chromosome ends. In this review we present an overview on the chromatin organization in lower and higher eukaryotes, and discuss the recent results on the peculiar features of telomeric nucleosomes and on the epigenetic status of mammalian telomeres.

Structural biology of shelterin and telomeric chromatin: the pieces and an unfinished puzzle

The six-subunit shelterin complex binds to mammalian telomeres and protects them from triggering multiple DNA damage response pathways. The loss of this protective function by shelterin can have detrimental effects on cells. In this review, we first discuss structural studies of shelterin, detailing the contributions of each subunit and inter-subunit interactions in protecting chromosome ends. We then examine the influence of telomeric chromatin dynamics on the function of shelterin at telomeres. These studies provide valuable insights and underscore the challenges that future research must tackle to attain high-resolution structures of shelterin.