Human RAP1 specifically protects telomeres of senescent cells from DNA damage.

Repressor/activator protein 1 (RAP1) is a highly evolutionarily conserved protein found at telomeres. Although yeast Rap1 is a key telomere capping protein preventing non‐homologous end joining (NHEJ) and consequently telomere fusions, its role at mammalian telomeres in vivo is still controversial. Here, we demonstrate that RAP1 is required to protect telomeres in replicative senescent human cells. Downregulation of RAP1 in these cells, but not in young or dividing pre‐senescent cells, leads to telomere uncapping and fusions. The anti‐fusion effect of RAP1 was further explored in a HeLa cell line where RAP1 expression was depleted through an inducible CRISPR/Cas9 strategy. Depletion of RAP1 in these cells gives rise to telomere fusions only when telomerase is inhibited. We further show that the fusions triggered by RAP1 loss are dependent upon DNA ligase IV. We conclude that human RAP1 is specifically involved in protecting critically short telomeres. This has important implications for the functions of telomeres in senescent cells. Synopsis: Human RAP1 specifically protects critically short telomeres from DNA damage response activation and telomere fusions in replicative senescent fibroblasts and tumor cells upon telomerase inhibition. RAP1 protects critically short telomeres of replicative senescent cells and HeLa cells upon telomerase inhibition.Telomere fusions generated by RAP1 depletion, are dependent on DNA ligase IV.RAP1 downregulation compromises re‐growth of replicative senescent cells upon p21 inhibition. [ABSTRACT FROM AUTHOR]