DNA processing is not required for ATM-mediated telomere damage response after TRF2 deletion

Abstract

Telomere attrition and other forms of telomere damage can activate the ATM kinase pathway. What generates the DNA damage signal at mammalian chromosome ends or at other double-strand breaks is not known. Telomere dysfunction is often accompanied by disappearance of the 3' telomeric overhang, raising the possibility that DNA degradation could generate the structure that signals. Here we address these issues by studying telomere structure after conditional deletion of mouse TRF2, the protective factor at telomeres. Upon removal of TRF2 from TRF2(F/-) p53-/- mouse embryo fibroblasts, a telomere damage response is observed at most chromosome ends. As expected, the telomeres lose the 3' overhang and are processed by the non-homologous end-joining pathway. Non-homologous end joining of telomeres was abrogated in DNA ligase IV-deficient (Lig4-/-) cells. Unexpectedly, the telomeres of TRF2-/- Lig4-/- p53-/- cells persisted in a free state without undergoing detectable DNA degradation. Notably, the telomeres retained their 3' overhangs, but they were recognized as sites of DNA damage, accumulating the DNA damage response factors 53BP1 and gamma-H2AX, and activating the ATM kinase. Thus, activation of the ATM kinase pathway at chromosome ends does not require overhang degradation or other overt DNA processing.

The significance of telomeric aggregates in the interphase nuclei of tumor cells

“…However, this has not been demonstrated in tumor models. We know from studies of de Lange and colleagues that the absence of TRF2 leads to the formation of telomeric fusions which lead to cell death and senescence [van Steensel et al, 1998;Celli and de Lange, 2005]. Whether these fusions involve TA formation and can be linked to genomic instability and cancer has not been investigated.…”

Single strand DNA binding proteins 1 and 2 protect newly replicated telomeres

“…mSSB1 ∆/∆ MEFs show a significant decrease in the number of end-to-end chromosome fusions after removal of TRF2, due to decreased activation of the ATM-Chk2 signaling pathway essential for C-NHEJ-mediated repair of telomeres [24,28]. In addition, mSSB1 and mSSB2 function to protect newly replicated Goverhangs.…”

Telomere attrition induces a DNA double-strand break damage signal that reactivates p53 transcription in HTLV-I leukemic cells

“…Critically short telomeres and telomeres unprotected by the telomere-binding protein complex or shelterin are recognized and processed as DNA double-strand break (DSB) (Celli et al, 2006) and become sites for recruitment of numerous proteins involved in DNA damage responses, referred as telomere dysfunctioninduced foci (TIF) (Celli and de Lange, 2005), leading to activation of the ataxia telangiectasia mutated (ATM) kinase pathway. Activation of ATM stimulates Chk2, which phosphorylates p53 on serine-20, disrupting its interaction with Mdm2 and stabilizing p53 protein in response to DNA damage (Shieh et al, 1997;Chehab et al, 2000b).…”