Eukaryotic cells evolved telomeres, specialized nucleoproteic complexes, to protect and replicate chromosome ends. In most organisms, telomeres consist of short, repetitive G-rich sequences added to chromosome ends by a reverse transcriptase, called telomerase. Specific DNA-binding protein complexes associate with telomeric sequences allowing cells to distinguish chromosome ends from sites of DNA damage. When telomeres become dysfunctional, either through excessive shortening or due to loss of proteins that preserve their function, they eventually lead to the activation of cell cycle checkpoints and/or to end-to-end fusion that ultimately give rise to chromosome breakage during anaphase. These events that in mammals might lead to several diseases including cancer clearly indicate that telomeres play critical roles in the maintenance of genome stability. Emerging data suggest a functional link between epigenetic marks and telomere homeostasis although most mechanisms underlying this connection are not known. Our preliminary results on the telomeric role of either Drosophila Pendolino (a conserved E2 variant ubiquitin-conjugating enzyme) or Eff, the fly ortholog of UbcH5B ubiquitin-conjugating enzyme in promoting histone methylation and Heterochromatin Protein 1 (HP1) ubiquitination, respectively, suggest that these proteins are indeed good candidates for the study of the conserved epigenetic regulation of telomere maintenance. In this regard, we propose to use D. melanogaster and A. thaliana, two model systems in which the epigenetics of telomere is well established, as well as human cells to tackle 3 main tasks: Assessing the roles of Peo (a) and its human ortholog AKTIP (b) in histone methylation at telomeres; (c) Studying the role of Eff/UbcH5B in the regulation of telomere protection in Drosophila, mammalian and plants. We believe that our results will highlight new pathways in the epigenetic regulation of telomere protection also for humans.
Telomere dysfunction is one of the major sources of tumor-associated genomic instability. Loss of telomeric DNA generates karyotypic instability in many tumors, accompanied by amplification and deletion of chromosomal segments (16). Extensively eroded telomeres are sensed as DNA breaks, leading to the activation of cell cycle checkpoints and inappropriate DNA repair, which may result in end-to-end fusion (TFs) (16). It has been suggested that TFs are the consequence of telomere shortening that occurs in early stages of carcinogenesis, and that short telomeres are fusigenic because they cannot recruit sufficient amount of telomere capping proteins (16,17).
In line with telomere dysfunction as a driver of genome instability, disruption of the telomere-capping factor TRF2 in primary human fibroblasts has been shown to promote the formation of non-reciprocal translocations, very likely through a breakage-fusion-bridge (BFB) mechanism. The genomic alterations resulting from these BFB cycles, including deletions and amplifications of chromosomal segments increase the mutability of the genome, thereby accelerating acquisition of mutant oncogenes and loss of tumor suppressor genes (16,17). Despite the importance in cancer biology, the mechanisms underlying telomere capping are not fully understood, and a complete repertoire of telomere-associated proteins remains to be defined. The discovery of a new pathway on the regulation of telomere maintenance is a relatively rare event, as in the past decade these factors have been discovered at a rate of approximately one a year.
As chromatin structures, telomeres pose targets for epigenetic molecular mechanisms including DNA methylation, histone modification, chromatin remodeling or RNAi. In the case of telomeres, however, the ultimate result of these processes is not a change in expression of a target gene, but rather a change of telomere length or structure, which may be reflected in modulation of telomere protective function. However the molecular bases of this epigenetic regulation remain still elusive. Our preliminary data on the telomeric role of Pendolino/AKTIP and Eff/UbcH5B in promoting histone methylation and HP1 ubiquitination, respectively, strongly suggest that these proteins could represent candidate factors for the epigenetic regulation of telomere maintenance. Thus, by further addressing the role of these factors in the model organisms Drosophila melanogaster and Arabidopsis thaliana, in which the link between epigenetics and telomere biology is well addressed, will improve our advancement in the identification of new evolutionary conserved pathways of telomere protections.
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