Multiple roles for MRE11 at uncapped telomeres

Abstract

The ends of linear eukaryotic chromosomes are capped by sequences known as telomeres. Although these are essentially one half of a DNA double-strand break, which is a pathogenic lesion that must be repaired to maintain the genome's integrity, telomeres do not normally activate DNA damage repair pathways. A major player in the process of telomere maintenance is the MRN complex, made up of three proteins (MRE11, RAD50 and NBS1). Now a study in mice using alleles that inactivate either the whole MRN complex or just the nuclease activity of MRE1, shows that MRE11 serves two functions at the telomere. It protects newly synthesized telomeric ends from repair factors by promoting the formation of an overhanging DNA end, and it degrades the overhang to promote fusion repair when the telomere is not functioning properly. The ends of linear eukaryotic chromosomes are capped by sequences known as telomeres. Although telomeres are essentially one half of a DNA double-strand break, which is a pathogenic lesion that must be repaired, telomeres do not normally activate DNA damage repair pathways. Here, the three-member MRN complex is shown to serve two roles at the telomere: it protects newly synthesized telomeric ends from repair factors and it promotes a type of fusion repair when the telomere is not functioning properly. Progressive telomere attrition or uncapping of the shelterin complex elicits a DNA damage response as a result of a cell’s inability to distinguish dysfunctional telomeric ends from DNA double-strand breaks1. Telomere deprotection activates both ataxia telangiectasia mutated (ATM) and telangiectasia and Rad3-related (ATR) kinase-dependent DNA damage response pathways, and promotes efficient non-homologous end-joining (NHEJ) of dysfunctional telomeres2,3,4,5. The mammalian MRE11–RAD50–NBS1 (MRN; NBS1 is also known as NBN) complex interacts with ATM to sense chromosomal double-strand breaks and coordinate global DNA damage responses6,7. Although the MRN complex accumulates at dysfunctional telomeres, it is not known whether mammalian MRN promotes repair at these sites. Here we address this question by using mouse alleles that either inactivate the entire MRN complex or eliminate only the nuclease activities of MRE11 (ref. 8). We show that cells lacking MRN do not activate ATM when telomeric repeat binding factor 2 (TRF2) is removed from telomeres, and ligase 4 (LIG4)-dependent chromosome end-to-end fusions are markedly reduced. Residual chromatid fusions involve only telomeres generated by leading strand synthesis. Notably, although cells deficient for MRE11 nuclease activity efficiently activate ATM and recruit 53BP1 (also known as TP53BP1) to deprotected telomeres, the 3′ telomeric overhang persists to prevent NHEJ-mediated chromosomal fusions. Removal of shelterin proteins that protect the 3′ overhang in the setting of MRE11 nuclease deficiency restores LIG4-dependent chromosome fusions. Our data indicate a critical role for the MRN complex in sensing dysfunctional telomeres, and show that in the absence of TRF2, MRE11 nuclease activity removes the 3′ telomeric overhang to promote chromosome fusions. MRE11 can also protect newly replicated leading strand telomeres from NHEJ by promoting 5′ strand resection to generate POT1a–TPP1-bound 3′ overhangs.