Telomeres of all eukaryotes are nucleoprotein complexes that protect the extremities of linear chromosomes from degradation and fusion, counterbalance incomplete replication of terminal DNA, and maintain genome stability. Drosophila telomeres are elongated by targeted transposition of specialized retroelements rather than telomerase activity. They are capped independently of the terminal DNA sequence by terminin, a complex of non-conserved fast-evolving proteins that is functionally analogous to human shelterin. Fly telomeres are also capped by conserved non-terminin proteins, most of which have human counterparts. We have hypothesized that after telomerase loss Drosophila rapidly evolved terminin to protect chromosome ends in a sequence independent fashion and that non-terminin proteins correspond to ancestral telomere-associated proteins, with human homologues possibly involved in telomere maintenance.
This proposal is aimed at a further functional characterization of one non-terminin Drosophila protein (Peo) and its human orthologue AKTIP. Peo and AKTIP are E2 variant ubiquitin conjugating enzymes. Mutations in peo reduce methylation of histone H3 lysine 9 (H3K9) and cause end-to-end fusions that preferentially involve telomeres juxtaposed to heterochromatin, while AKTIP depletion results in defective telomere replication. The experiments we propose are aimed at defining the role of these proteins at fly and human telomeres. In addition, we will exploit our Co-IP/mass spectrometry findings to identify novel terminin and non-terminin proteins required for Drosophila telomere capping; the non-terminin proteins identified in this way are likely to have human homologues required for telomere maintenance. We believe that the parallel characterization of these proteins in both flies and humans will generate synergistic information that will help deciphering their respective functions at telomeres.
Telomeres play important roles in carcinogenesis. Critically short telomeres or improperly capped telomeres are sensed as DNA breaks leading to cell cycle arrest and cell senescence. Thus, most cancers reactivate telomerase, which is normally silent in somatic cells, to sustain cell proliferation. Short or uncapped telomeres are also subjected to inappropriate DNA repair leading to telomere fusions (TFs). The dicentric chromosomes generated by TFs can cause chromosome breakage during anaphase and chromothripsis, promoting tumor development [32-34]. In addition, mutation or dysregulation of genes encoding telomere maintenance proteins can lead to cancer predisposition syndromes [35, 36]. As chromatin structures, telomeres pose targets for epigenetic molecular mechanisms including DNA methylation, histone modification, chromatin remodeling or RNAi. The formation of higher-order chromatin domains is recognized to be essential for dosage compensation, recombination, chromosome condensation and segregation, and also for telomere function.
Despite the importance in cancer biology, the mechanisms underlying telomere capping are not fully understood and the identification of factors contributing to the plasticity of heterochromatin structure are still elusive.
This grant proposal is aimed at the identification and characterization of proteins involved in human telomere maintenance exploiting Drosophila as model system.
As chromatin structures, telomeres pose targets for epigenetic molecular mechanisms including DNA methylation, histone modification, chromatin remodeling or RNAi. The formation of higher-order chromatin domains is recognized to be essential for dosage compensation, recombination, chromosome condensation and segregation, and also for telomere function. However, the factors that contribute to the plasticity of heterochromatin structure are elusive. We believe that the characterization of telomere factors, such as Peo/Aktip, in controlling the epigenetic transition between euchromatin and heterochromatin may shed light on the connection between chromatin regulation and telomere maintenance. Any progress in the understanding of human telomere biology that is likely to have an impact on cancer prevention and therapy.