DNA Ligase IV-Dependent NHEJ of Deprotected Mammalian Telomeres in G1 and G2

Background: Telomeres are required to prevent end-to-end chromosome fusions. End-to-end fusions of metaphase chromosomes are observed in mammalian cells with dysfunctional telomeres due to diminished function of telomere-associated proteins and in cells experiencing extensive attrition of telomeric DNA. However, the molecular nature of these fusions and the mechanism by which they occur have not been elucidated. Results: We document that telomere fusions resulting from inhibition of the telomere-protective factor TRF2 are generated by DNA ligase IV-dependent nonhomologous end joining (NHEJ). NHEJ gives rise to covalent ligation of the C strand of one telomere to the G strand of another. Breakage of the resulting dicentric chromosomes results in nonreciprocal translocations, a hallmark of human cancer. Telomere NHEJ took place before and after DNA replication, and both sister telomeres participated in the reaction. Telomere fusions were accompanied by active degradation of the 3′ telomeric overhangs. Conclusions: The main threat to dysfunctional mammalian telomeres is degradation of the 3′ overhang and subsequent telomere end-joining by DNA ligase IV. The involvement of NHEJ in telomere fusions is paradoxical since the NHEJ factors Ku70/80 and DNA-PKcs are present at telomeres and protect chromosome ends from fusion.