Human RAP1 specifically protects telomeres of senescent cells from DNA damage

Repressor/activator protein 1 (RAP1) is a highly evolutionary conserved protein found at telomeres. Here we demonstrate that RAP1 is required to protect telomeres specifically in replicative senescent human cells. Downregulation of RAP1 in the primary lung fibroblast cell line MRC-5, but not in young or dividing pre-senescent cells, leads to telomere uncapping and chromosome fusions. We have also found that the fusions triggered by RAP1 loss are dependent upon ligase IV, indicating that they are generated through the classical NHEJ between telomere ends. We conclude that the human RAP1 is specifically involved in protecting critically short telomeres. Overall design: we PCR amplified chromosome fusions of senescence cells transduced with an shControl or an shRAP1 expressing plasmid and we used Nanopore sequencing to gain insights into the type of fusion events.