Acute telomere deprotection prevents ongoing BFB cycles and rampant instability in p16 INK4a -deficient epithelial cells

// Aina Bernal 1 , Marc Molto-Abad 1, 2 , Daniel Dominguez 1 and Laura Tusell 1 1 Unitat de Biologia Cel·lular, Facultat de Biociencies, Universitat Autonoma de Barcelona, 08193 Cerdanyola del Valles, Spain 2 Current address: Unitat de Malalties Minoritaries, Hospital Universitari de la Vall d’Hebron, 08035 Barcelona, Spain Correspondence to: Laura Tusell, email: laura.tusell@uab.cat Keywords: MCF-10A; breast epithelial cells; chromosome instability; telomere-dysfunction; TRF2 ΔBΔM Received: June 28, 2017     Accepted: May 13, 2018     Published: June 05, 2018 ABSTRACT Telomere dysfunction drives chromosome instability through endless breakage-fusion-bridge (BFB) cycles that promote the formation of highly rearranged genomes. However, reactivation of telomerase or ALT-pathway is required for genome stabilisation and full malignant transformation. To allow the unrestricted proliferation of cells at risk of transformation, we have established a conditional system of telomere deprotection in p16 INK4a -deficient MCF-10A cells with modified checkpoints. After sustained expression of a dominant negative form of the shelterin protein TRF2 (TRF2 ΔBΔM ), cells with telomere fusion did progress to anaphase but no signs of ongoing BFB cycles were observed, thus anticipating proliferation defects. Indeed, 96 h TRF2 ΔBΔM expression resulted in noticeable growth proliferation defects in the absence of cell cycle disturbances. Further transient periods of 96 h telomere uncapping did not result in cell cycle disturbances either. And reduction of the telomere damage to short acute deprotection periods did not in any case engender cells with a reorganised karyotype. Strikingly, the growth arrest imposed in cells showing dysfunctional telomeres was not accompanied by an activation of the DNA damage response at cellular level, or by the presence of visible markers of senescence or apoptosis. We propose that the deprotection of many telomeres simultaneously, even for a short time, results in a local activation of the cellular stress response which consequently triggers gradual cell withdrawal from cell cycle, restraining the onset of genomic instability.