TERRA R-loops connect and protect sister telomeres in mitosis

Abstract Resolution of cohesion between sister telomeres in human cells depends on TRF1-mediated recruitment of the polyADP-ribosyltransferase, tankyrase to telomeres. In cells where tankyrase is deleted or the tankyrase binding site in TRF1 is mutated, sister telomeres remain cohered in mitosis. Human aged cells and ALT cancer cells naturally exhibit persistent telomere cohesion due to shortened telomeres that do not recruit sufficient TRF1/tankyrase for resolution. Persistent cohesion plays a protective role, but the mechanism by which sister telomeres remain cohered is not well understood. Here we show that telomere repeat containing RNA (TERRA) holds sister telomeres together through RNA-DNA hybrid (R-loop) structures. We show that a tankyrase-interacting partner, the RNA-binding protein C19orf43 is required for resolution of telomere cohesion and for repression of TERRA R-loops. Depletion of C19orf43 led to persistent telomere cohesion and an increase in TERRA R-loops. Overexpression of RNaseH1 counteracted persistent cohesion in C19orf43-depleted cells, as well as in aged and ALT cells. In fact, treatment of cohered telomeres in mitotic cells with RNaseH1 in situ, was sufficient to resolve sister telomere cohesion, confirming that RNA-DNA hybrids hold sister telomeres together. Consistent with a protective role for persistent telomere cohesion, depletion of C19orf43 in aged cells reduced DNA damage and significantly delayed replicative senescence. We propose that the inherent inability of shortened telomeres to recruit R-loop repressing machinery permits a controlled onset of senescence.