Exo1 plays a major role in DNA end resection in humans and influences double-strand break repair and damage signaling decisions

Department of Radiation Oncology, University of Texas Southwestern Medical Center, Dallas, USA.
DNA repair (Impact Factor: 3.11). 02/2012; 11(4):441-8. DOI: 10.1016/j.dnarep.2012.01.006
Source: PubMed


The resection of DNA double-strand breaks (DSBs) to generate ssDNA tails is a pivotal event in the cellular response to these breaks. In the two-step model of resection, primarily elucidated in yeast, initial resection by Mre11-CtIP is followed by extensive resection by two distinct pathways involving Exo1 or BLM/WRN-Dna2. However, resection pathways and their exact contributions in humans in vivo are not as clearly worked out as in yeast. Here, we examined the contribution of Exo1 to DNA end resection in humans in vivo in response to ionizing radiation (IR) and its relationship with other resection pathways (Mre11-CtIP or BLM/WRN). We find that Exo1 plays a predominant role in resection in human cells along with an alternate pathway dependent on WRN. While Mre11 and CtIP stimulate resection in human cells, they are not absolutely required for this process and Exo1 can function in resection even in the absence of Mre11-CtIP. Interestingly, the recruitment of Exo1 to DNA breaks appears to be inhibited by the NHEJ protein Ku80, and the higher level of resection that occurs upon siRNA-mediated depletion of Ku80 is dependent on Exo1. In addition, Exo1 may be regulated by 53BP1 and Brca1, and the restoration of resection in BRCA1-deficient cells upon depletion of 53BP1 is dependent on Exo1. Finally, we find that Exo1-mediated resection facilitates a transition from ATM- to ATR-mediated cell cycle checkpoint signaling. Our results identify Exo1 as a key mediator of DNA end resection and DSB repair and damage signaling decisions in human cells.

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    • "Collectively, these data indicated that K- H plays a role in facilitating specific DSB repair processes. The absence of radial chromosomes in K-H-depleted cells, commonly seen in cells lacking HR-mediated DSB repair (Cheung et al., 2002; Tomimatsu et al., 2012), suggested that the primary DSB repair defect in K-H-deficient cells was NHEJ. However, potential defects in HR are still being delineated as these assays are not entirely definitive to rule out HR. "
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    • "Although Fun30 facilitates both extensive resection mechanisms, the phenotype of fun30D is similar to exo1D and overexpression of Exo1 suppresses the DNA damage sensitivity of the fun30D mutant (Chen et al. 2012; Costelloe et al. 2012). Additionally, SMARCAD1, the human ortholog of Fun30, is required for RPA localization to laser-induced DNA damage, similar to the role of EXO1 (Costelloe et al. 2012; Tomimatsu et al. 2012). Although the recruitment of RSC, INO80, and Fun30 would be expected to precede resection, localization of these factors to DSBs is reduced in exo1D sgs1D cells; furthermore, recruitment of Sgs1, Dna2, and Exo1 is reduced in the fun30D mutant, indicating a complex interdependency. "
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