Replicative crisis is a senescence-independent process that acts as a final barrier against oncogenic transformation by eliminating pre-cancerous cells with disrupted cell cycle checkpoints1. It functions as a potent tumour suppressor and culminates in extensive cell death. Cells rarely evade elimination and evolve towards malignancy, but the mechanisms that underlie cell death in crisis are not well understood. Here we show that macroautophagy has a dominant role in the death of fibroblasts and epithelial cells during crisis. Activation of autophagy is critical for cell death, as its suppression promoted bypass of crisis, continued proliferation and accumulation of genome instability. Telomere dysfunction specifically triggers autophagy, implicating a telomere-driven autophagy pathway that is not induced by intrachromosomal breaks. Telomeric DNA damage generates cytosolic DNA species with fragile nuclear envelopes that undergo spontaneous disruption. The cytosolic chromatin fragments activate the cGAS-STING (cyclic GMP-AMP synthase-stimulator of interferon genes) pathway and engage the autophagy machinery. Our data suggest that autophagy is an integral component of the tumour suppressive crisis mechanism and that loss of autophagy function is required for the initiation of cancer.

Classical non-homologous end joining 1 (cNHEJ) and homologous recombination 2 (HR) compete for the repair of double stranded breaks of DNA during the cell cycle. HR is inhibited in G1 phase of the cell cycle, but both pathways are active in S and G2 phases. Why cNHEJ does not always outcompete HR in S and G2 phases has been unclear. Here we show that CYREN is a cell cycle specific inhibitor of cNHEJ. CYREN suppression allows cNHEJ at telomeres and intrachromosomal breaks during S and G2 phases, while cells lacking CYREN accumulate chromosomal aberrations upon damage induction, specifically outside G1 phase. CYREN acts by binding to the Ku70/80 heterodimer and preferentially inhibits cNHEJ at breaks with overhangs by protecting them. We therefore propose that CYREN is a direct cell cycle inhibitor of cNHEJ, thereby promoting error free repair by HR in cell cycle phases where sister chromatids are present.

Cancers arise through the accumulation of genetic and epigenetic alterations that enable cells to evade telomere-based proliferative barriers and achieve immortality. One such barrier is replicative crisis—an autophagy-dependent program that eliminates checkpoint-deficient cells with unstable telomeres and other cancer-relevant chromosomal aberrations1,2. However, little is known about the molecular events that regulate the onset of this important tumour-suppressive barrier. Here we identified the innate immune sensor Z-DNA binding protein 1 (ZBP1) as a regulator of the crisis program. A crisis-associated isoform of ZBP1 is induced by the cGAS–STING DNA-sensing pathway, but reaches full activation only when associated with telomeric-repeat-containing RNA (TERRA) transcripts that are synthesized from dysfunctional telomeres. TERRA-bound ZBP1 oligomerizes into filaments on the outer mitochondrial membrane of a subset of mitochondria, where it activates the innate immune adapter protein mitochondrial antiviral-signalling protein (MAVS). We propose that these oligomerization properties of ZBP1 serve as a signal amplification mechanism, where few TERRA–ZBP1 interactions are sufficient to launch a detrimental MAVS-dependent interferon response. Our study reveals a mechanism for telomere-mediated tumour suppression, whereby dysfunctional telomeres activate innate immune responses through mitochondrial TERRA–ZBP1 complexes to eliminate cells destined for neoplastic transformation.

These findings disclose an innovative, non-invasive strategy to induce therapeutic neovascularization in a mouse model of HLI, emerging as a novel approach in the treatment of critical limb ischemia patients.