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Telomeric double strand breaks undergo resection - but not repair - in G1 human cells




Nelson, Christopher Boulanger, author
Bailey, Susan, advisor
Argueso, Lucas, committee member
Kato, Takamitsu, committee member
Wiese, Claudia, committee member
Miller, Benjamin, committee member
Chicco, Adam, committee member

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Telomeres are specialized G-rich repetitive regions at the ends of eukaryotic chromosomes (TTAGGGn in mammalian cells). Telomeres function to prevent double strand break (DSB) repair activities at chromosome ends, in order to avoid fusion events which result in lethal dicentric chromosomes. Telomeric repeats make up an appreciable amount of genomic DNA (1-15kb per chromosome end). Therefore, an interesting question becomes, how is the inevitable DSB occurring within a telomere dealt with by the cell? It has been suggested that DSBs within telomeric DNA may not be repaired at all, as DSB DNA damage response (DDR) foci at telomeres do not resolve following large amounts of global DNA damage (e.g. ionizing radiation). Such studies also suggest that telomere repair may be inhibited specifically in G1, as the majority of surviving cells with unresolved telomere damage responses were senescent (a G1 phenotype). On the other hand, studies on the fragmentation of telomeric DNA following cutting with a telomere-targeted endonucleases indicate that repair of telomere-specific DSBs involves Homologous Recombination (HR) and Break-Induced Replication (BIR). However, a marker of telomeric DSB DDRs was only observed in cells with BrdU incorporation, in support of the view that repair of telomeric DSBs is an S/G2-related process, which does not occur in G1. To follow up on these studies, we investigated telomeric DDRs and DSB repair in individual G1 cells using ionizing radiation (IR) and a targeted telomere-cutting endonuclease. IR exposure could potentially induce loss of telomere function, such that persistent DDRs may not represent actual DSBs. To rule out this possibility, we evaluated whether persistent telomeric DDRs following IR occurred at telomeres that were critically short or lacking TRF2. We found that persistent telomeric DDRs occurred at telomeres of normal length and TRF2 status, in support of the conclusion that G1 telomeric DSBs are irreparable. Additionally, using the telomere-targeted endonuclease we observed that telomeric DSBs in G1 cells elicited a relatively conventional DSB DDR – with one important exception – G1 telomeric DDRs failed to recruit 53BP1, an event implicated in the completion of DSB repair by most pathways, but especially, canonical non-homologous end joining (cNHEJ). Further, shRNA knockdown and kinase inhibition of the cNHEJ factor DNA-PKcs, provided evidence that cNHEJ is not responsible for repair of telomeric DSBs, and that DNA-PKcs does not influence recruitment of 53BP1 to telomeric DSBs in G1. Partial deprotection of telomeres, achieved by siRNA depletion of TRF2, also failed to alleviate inhibition of 53BP1 recruitment to G1 telomeric DSBs, suggesting that 53BP1 recruitment to telomeric DSBs may require full deprotection of telomeres. However, as 53BP1 recruitment occurs at de-protected telomeres, this idea would be difficult to test. Most likely related to the lack of 53BP1 recruitment, an abundance of bidirectionally occurring single-stranded DNA was observed at G1 telomeric DSBs, a characteristic of long-range repair-associated resection. In support of long-range resection, RPA70 and phospho-RPA32 were observed at G1 telomeric DSBs. Additionally, conventional DSB repair-associated resection machinery, including MRE11 and EXO1, but not the telomere processing exonuclease Apollo, promoted resection at telomeric DSBs. We then investigated whether long-range resection-dependent repair was occurring at G1 telomeric DSBs via RAD51 or RAD52 foci, and DNA synthesis (S/G2 related processes). Despite activity resembling long-range repair-associated resection at G1 telomeric DSBs, no evidence for repair by these pathways was found. Taken together, the results presented here provide strong evidence in support of the view that telomeric DSBs in G1 are unrepairable. Therefore, the extensive resection observed at telomeric DSBs must be reflective of an alternative, non-repair related function, perhaps related to structural end-protection. We speculate that resection at G1 telomeric DSBs may serve to prevent 53BP1 recruitment, thereby circumventing a full DDR and activation of cNHEJ, a scenario that would create a serious threat to genome stability.


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