Telomere entanglements: formation, disentanglement, and impact of failed disentanglement on genome stability

About This Grant

Abstract Telomere entanglements: formation, disentanglement, and impact of failed disentanglement on genome stability To preserve genome stability, replicated sister chromatids must be disentangled and cleanly separated at mitosis. Telomeres safeguard this process by protecting chromosome ends from degradation and end-joining reactions that generate dicentric chromosomes and rampant chromosome instability. In most human somatic cells, however, telomerase is inactive, leading to progressive telomere erosion over time. This raises a fundamental question: what protective functions are lost as telomeres shorten and become dysfunctional? Telomeres are well known for preventing end fusions, but we have uncovered an additional threat: telomere entanglements. These arise when stalled replication forks at dysfunctional telomeres fail to restart and engage in aberrant interactions, leading to persistent DNA bridges during mitosis. The fission yeast telomere-binding protein Taz1, and its mammalian ortholog TRF1, promote replication fork progression through telomeres and prevent such entanglements. Loss of Taz1 causes stalled telomeric forks that generate anaphase-spanning DNA bridges. We find that resolution of these structures depends on the timing of anaphase midregion nuclear envelope breakdown, which exposes the entanglements to the cytoplasm, an unexpected but essential step for entanglement resolution. This proposal dissects the mechanisms governing telomere entanglement formation and resolution. We hypothesize that the most problematic entanglements stem from strand invasions between stalled forks on different chromosomes, forming non-sister telomere entanglements. We will define their molecular structure and investigate how long noncoding telomeric RNAs and RecQ helicases contribute to their formation. We further show that resolution involves a noncanonical function of Topoisomerase II, likely modulated by the condensation state of Top2- DNA complexes, a novel concept with broad implications. While these discoveries emerged from studies in S. pombe, our preliminary findings reveal similar entanglement phenotypes in mammalian cells, particularly in response to telomeric replication stress. We will extend these studies to human cells undergoing telomere-driven replicative aging, to assess whether telomere entanglements contribute to the genomic instability associated with aging. Together, this work defines a previously unrecognized consequence of telomere dysfunction and illuminates the molecular handoff between stalled replication and chromosome segregation at the critical final act of mitosis, the moment of truth when euploidy is either preserved or lost.