A role for RNA in sister chromatid cohesion at human telomeres

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Project Summary Telomeres, specialized structures comprised of TTAGGG repeats and the six-subunit shelterin complex, require distinct mechanisms for their protection, replication, and cohesion. Following replication in S phase, sister chromatids are held together (cohered) until mitosis when they are distributed to daughter cells. Resolution of cohesion between sister telomeres requires the poly(ADP-ribose) polymerase, tankyrase 1. Tankyrase localizes to telomeres by binding to the TTAGGG-repeat binding shelterin subunit TRF1, in late S/G2 to resolve cohesion. In tankyrase 1 depleted cells sister telomeres remain cohered in mitosis (termed persistent telomere cohesion), despite normal resolution of sister chromatid arms and centromeres. This proposal builds on our recent discovery that persistent telomere cohesion serves a protective function (in the absence of telomerase) at critically short telomeres. We found that reduction of TRF1 (and the consequent tankyrase deficiency) at shortened telomeres in aged cells (or telomeres negative cancer cells) prevents resolution of cohesion and that this (unexpectedly) serves a protective role to prevent premature senescence or growth arrest. We will elucidate the mechanisms, which we show are RNA-based. Our identification of RNA-binding proteins as a significant group of tankyrase interactors in a proteomic screen prompted an investigation into a function for RNA in cohesion. Indeed, we have uncovered a role for the long non-coding telomere repeat-containing RNA (TERRA) in persistent telomere cohesion. We show that accumulation of TERRA and TERRA R-loops (RNA/DNA hybrids) occurs concomitant with persistent telomere cohesion to protect critically shortened telomeres. We will elucidate the proteins and pathways required to reduce TERRA levels for resolution of telomere cohesion, determine their role in ensuring protection of short telomeres in presenescent cells, and explore a potential phase transition mechanism used by tankyrase to resolve telomere cohesion. Our study provides a unifying and novel mechanism that relies on RNA, R-loops, and persistent telomere cohesion to buffer shortened telomeres and promote genome integrity.