Abstract B63: The role of the telomere-associated protein TRF2 in the DNA damage response

Abstract

Abstract Telomeres act as protective caps to disguise chromosome ends from being recognized as a DNA double-strand break (DSB). Research covering the last decade from our group and other groups has shown that several proteins known to play important roles in the DNA damage response (DDR), especially in the non-homologous end-joining (NHEJ) pathway, also have critical roles in telomere maintenance. Interestingly, within this collection of DDR proteins, which have critical roles at the telomere, are two important protein kinases, ataxia telangiectasia mutated protein kinase (ATM) and the DNA-dependent protein kinase catalytic subunit (DNA-PKcs). These findings led us to study whether certain telomere-associated proteins are substrates for these kinases, and if so, precisely which function(s) are facilitated by these modifications. We initially reported that the telomere-associated protein TRF2 is rapidly and transiently phosphorylated in response to DNA damage by an ATM dependent pathway. TRF2 is an essential mammalian telomere protein that binds directly to telomeric DNA and acts as a keystone for anchoring additional proteins at the telomere. Furthermore, we found that the DNA damage-induced phosphorylated form of TRF2 is not bound to telomeric DNA in telomerase-dependent cell lines and rapidly accumulates at DNA damage sites. Our previous result implicated a role for DNA damage-induced phosphorylation of TRF2 in DNA repair, but did not provide direct functional evidence for TRF2 proteins involvement in the DDR. To reveal the functional role of DNA damage-induced phosphorylation of TRF2, we performed a classic disruption and compensatory mutational analysis of TRF2 at residue threonine188 (T188). We found that a disruption of T188 by substitution of an alanine residue essentially eliminates the DNA-PK-dependent fast pathway of DNA DSB repair, reduces clonigenic growth after X-ray treatment and alters γ-H2AX post-damage kinetics. Importantly, TRF2 protein containing a glutamic acid substitution at position 188, which would be expected to mimic a phosphorylated threonine, restored clonigenic growth after X-ray treatment, and partially restored DNA DSB repair and γ-H2AX post-damage kinetics. These results support the idea that DNA damage-induced phosphorylation of TRF2 is a critical component of the DDR. Importantly, our recent findings connect the highly transient DNA damage induction of human TRF2 phosphorylation to the DDR machinery via the NHEJ pathway. In addition to our findings, several other groups have reported evidence suggesting that TRF2 functions in the DDR. Therefore, crosstalk and sharing of protein components not only occurs between the DDR to the telomere, but also from the telomere to the DDR. DNA damage-induced phosphorylation of TRF2 may be required to facilitate the dissociation of TRF2 from the telomere to initiate its migration to sites of DNA damage and/or may function at DNA DSBs by modulating specific protein-protein interactions. Our current research focuses on determining whether additional proteins are shared between the DDR and the telomere, along with how these proteins are modulated functionally within these two different environments. Citation Information: Cancer Res 2009;69(23 Suppl):B63.