title: Spontaneous telomere to telomere fusions occur in unperturbed fission yeast cells.
abstract: Telomeres protect eukaryotic chromosomes from illegitimate end-to-end fusions. When this function fails, dicentric chromosomes are formed, triggering breakage-fusion-bridge cycles and genome instability. How efficient is this protection mechanism in normal cells is not fully understood. We created a positive selection assay aimed at capturing chromosome-end fusions in Schizosaccharomyces pombe. We placed telomere sequences with a head to head arrangement in an intron of a selectable marker contained on a plasmid. By linearizing the plasmid between the telomere sequences, we generated a stable mini-chromosome that fails to express the reporter gene. Whenever the ends of the mini-chromosome join, the marker gene is reconstituted and fusions are captured by direct selection. Using telomerase mutants, we recovered several fusion events that lacked telomere sequences. The end-joining reaction involved specific homologous subtelomeric sequences capable of forming hairpins, suggestive of ssDNA stabilization prior to fusing. These events occurred via microhomology-mediated end-joining (MMEJ)/single-strand annealing (SSA) repair and also required MRN/Ctp1. Strikingly, we were able to capture spontaneous telomere-to-telomere fusions in unperturbed cells. Similar to disruption of the telomere regulator Taz1/TRF2, end-joining reactions occurred via non-homologous end-joining (NHEJ) repair. Thus, telomeres undergo fusions prior to becoming critically short, possibly through transient deprotection. These dysfunction events induce chromosome instability and may underlie early tumourigenesis.
authors: Instituto Gulbenkian de Ciência, Rua da Quinta Grande 6, 2780-156 Oeiras, Portugal.
authors: Hugo
authors: H
authors: Almeida
authors: Miguel
authors: M
authors: Godinho Ferreira
mesh_terms: D043171:Chromosomal Instability; D015825:Chromosomes, Fungal / Q000235:genetics / Q000378:metabolism; D059766:DNA End-Joining Repair; D050939:Gene Fusion; D009369:Neoplasms / Q000235:genetics; D012568:Schizosaccharomyces / Q000235:genetics* / Q000378:metabolism; D029702:Schizosaccharomyces pombe Proteins / Q000235:genetics / Q000378:metabolism; D019098:Telomerase / Q000235:genetics / Q000378:metabolism; D016615:Telomere / Q000235:genetics / Q000378:metabolism*; D059505:Telomere Homeostasis; D034501:Telomere-Binding Proteins / Q000378:metabolism
chemical_list: C085979:RAD16 protein, S pombe; D029702:Schizosaccharomyces pombe Proteins; D034501:Telomere-Binding Proteins; C104344:taz1 protein, S pombe; C509188:trt1 protein, S pombe; D019098:Telomerase