Epigenetic Modifications Identified at Replication Stressed Telomeres

Persistent during DNA replication or inappropriately processed, telomere DNA secondary structures can have pathological consequences and are an established source of genome instability. Despite the advances on the role of DNA helicases and Shelterin proteins facilitating replication through telomeric secondary structures, the step-by-step analysis of induction of telomere fragility remain poorly understood. TRF1 is a key factor in facilitating replication at telomeres and we took advantage of its function to study the events happening at telomeres using an unbiased proteomic approach. Proteomic of Isolated Chromatin revealed that telomeric chromatin of TRF1 deficient cells resemble ALT telomeres with the recruitment of PML and also the enrichment of many ALT specific factors including ATRX, SMC5/6 complex. We also report for the first time that the high levels of recombination detected by CO-FISH in TRF1-/- cells is dependent on SMC5 and is in fact a conservative dependent DNA synthesis mediated by BIR and POLD3. Notably, we could not detect all hallmarks of ALT positive cells in our experimental system, including no heterogeneous telomere length and no c-circle formation.
Telomere fragility and Common Fragile Site share at least a common trigger, which is replication hindrance, both induced by aphidicolin treatment. Despite POLD3 dependent BIR being involved in CFS expression and now observed in our study at TRF1 depleted telomeres, we claim that this conservative DNA synthesis mechanism is not responsible for the induction of telomere fragility in TRF1 deficient cells. Therefore, all indicate that the mechanism generating telomere fragility and CFS are different.