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. This proposal is aimed at the identification and characterization of proteins involved in human telomere maintenance exploiting Drosophila as model system. 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 humans homologues possibly involved in telomere maintenance. In this proposal we will exploit data we recently obtained obtained from Co-IP/mass spectrometry experiments carried out in fly embryos, 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.
The soma and the germline markedly differ for the regulation of the DNA damage response and telomere regulation(Vermezovic et al., 2012). In human embryonic stem cells, telomerase is active and telomere length is appropriately set such that elongated telomeres can support cells divisions during development. However, telomerase and telomere elongation must be limited to ensure telomeres are short enough to limit cell proliferation in the adult to suppress cancer (Rivera et al., 2017).
Studies in human embryonic stem cells (hESCs) show that telomeres lengthen from the early blastocysts through early expansion, but they are stabilized at later passages(Zeng et al., 2014), suggesting that specific mechanisms have evolved to promote the optimal telomere length that ensures genomic stability .
Understanding how the regulation of telomere homeostasis is modulated in the soma and the germline in different organism is an open and challenging question. Drosophila offers an excellent opportunity to address this issue, because several genes appear to be specifically required for telomere maintenance in the embryo or in the germline are dispensable at telomeres in somatic cells (our unpublished results). We believe that the proposed experiments will lead to the identification of (possible) additional components of the terminin complex, as well as novel non-terminin proteins required for telomere capping. These latter proteins are likely to have human homologues involved in telomere maintenance.
We believe that characterizing the role at telomeres of novel proteins that interact with the terminin components in fly embryos will help us shedding light into the specific requirements for the maintenance of germline and somatic telomeres.