TERRA increases at short telomeres in yeast survivors and regulates survivor associated senescence (SAS)

Cancer cells achieve immortality by employing either homology-directed repair (HDR) or the telomerase enzyme to maintain telomeres. ALT (alternative lengthening of telomeres) refers to the subset of cancer cells that employ HDR. ALT mechanisms are strongly conserved from yeast to human cells, with the yeast equivalent being referred to as survivors. The non-coding RNA, TERRA and its ability to form RNA-DNA hybrids have been implicated in ALT/survivor maintenance by promoting HDR. It is not understood which telomeres in ALT/survivors engage in HDR, nor is it clear which telomeres upregulate TERRA. Using yeast survivors as a model for ALT, we demonstrate that HDR only occurs at telomeres when they become critically short. Moreover, TERRA levels steadily increase as telomeres shorten and decrease again following HDR-mediated recombination. Surprisingly, we observe that survivors undergo cycles of senescence, in a similar manner to non-survivors following telomerase loss, which we refer to as survivor associated senescence (SAS). Similar to "normal" senescence, we observe that RNA-DNA hybrids slow the rate of SAS by decreasing the rate of telomere shortening. In summary, TERRA RNA-DNA hybrids regulate telomere dysfunction-induced senescence in both pre- and post-crisis cells.