Probing the Telomere Damage Response.

Telomere dysfunctions, rendered through replicative attrition of telomeric DNA or due to the removal of shelterin components, are recognized as DNA double-stranded breaks (DSBs) by the DNA damage repair (DDR) pathway. This leads to the activation of DNA damage checkpoint sensors, including the Mre11-Rad50-Nbs1 (MRN) complex, γ-H2AX and 53BP1, the ATM and ATR signal-transducing kinases, and downstream effectors, including Chk1, Chk2, and p53. Robust DNA damage response signals at dysfunctional telomeres, achieved by the complete deletion of TRF2 or by expressing dominant-negative mutant TPP1ΔRD, can be detected by their association with γ-H2AX and 53BP1 forming "telomere dysfunction induced foci (TIFs)." Induction of TIFs at telomeres provides an opportunity to quantify the extent of telomere dysfunction and monitor downstream signaling pathways.