The Role of ATM in the Deficiency in Nonhomologous End-Joining near Telomeres in a Human Cancer Cell Line

Abstract

Telomeres distinguish chromosome ends from double-strand breaks (DSBs) and prevent chromosome fusion. However, telomeres can also interfere with DNA repair, as shown by a deficiency in nonhomologous end joining (NHEJ) and an increase in large deletions at telomeric DSBs. The sensitivity of telomeric regions to DSBs is important in the cellular response to ionizing radiation and oncogene-induced replication stress, either by preventing cell division in normal cells, or by promoting chromosome instability in cancer cells. We have previously proposed that the telomeric protein TRF2 causes the sensitivity of telomeric regions to DSBs, either through its inhibition of ATM, or by promoting the processing of DSBs as though they are telomeres, which is independent of ATM. Our current study addresses the mechanism responsible for the deficiency in repair of DSBs near telomeres by combining assays for large deletions, NHEJ, small deletions, and gross chromosome rearrangements (GCRs) to compare the types of events resulting from DSBs at interstitial and telomeric DSBs. Our results confirm the sensitivity of telomeric regions to DSBs by demonstrating that the frequency of GCRs is greatly increased at DSBs near telomeres and that the role of ATM in DSB repair is very different at interstitial and telomeric DSBs. Unlike at interstitial DSBs, a deficiency in ATM decreases NHEJ and small deletions at telomeric DSBs, while it increases large deletions. These results strongly suggest that ATM is functional near telomeres and is involved in end protection at telomeric DSBs, but is not required for the extensive resection at telomeric DSBs. The results support our model in which the deficiency in DSB repair near telomeres is a result of ATM-independent processing of DSBs as though they are telomeres, leading to extensive resection, telomere loss, and GCRs involving alternative NHEJ.

Increased Mutagenic Joining of Enzymatically-Induced DNA Double-Strand Breaks in High-Charge and Energy Particle Irradiated Human Cells

“…Full details have been published elsewhere (11). It contains a transcriptional promoter, which after exposure to I-SceI, can be joined in cis to a promoter-less enhanced green fluorescent protein (eGFP) gene or in trans to a promoter-less Discosoma sp.…”

The DNA damage response at dysfunctional telomeres, and at interstitial and subtelomeric DNA double-strand breaks

“…In addition to large deletions, the increased processing of DSBs near telomeres also causes a high frequency of gross chromosome rearrangements. Gross chromosome rearrangements at subtelomeric DSBs occur via A-NHEJ, as shown by the presence of microhomology at the junctions of the chromosome fusions (Muraki et al, 2013). Although gross chromosome rearrangements often occur in combination with large deletions, they sometimes occur in their absence, demonstrating that large deletions and gross chromosome rearrangements can occur independently (Zschenker et al, 2009).…”

Crosstalk between telomere maintenance and radiation effects: A key player in the process of radiation-induced carcinogenesis

“…Both pathways have similar initial steps in commencing DNA repair, involving the binding of the MRE11/RAD50/NBS1 (MRN) complex to the site of the DSB, followed by activation of ATM. ATM then goes on to phosphorylate various proteins to recruit repair complexes to the site of the DSB, modify chromatin structure at the DSB to allow repair protein access, and activate cell cycle checkpoints to delay progression through the cell cycle until repair is complete [66]. Whether the cell undergoes repair via NHEJ or HR appears to depend on four proteins: RIF1 (Rap1-interacting factor 1), 53BP1 (p53 binding protein 1), BRCA1 (breast cancer type 1 susceptibility protein), and CtIP (C terminus-binding protein-interacting protein).…”