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.

1 Follower
56 Reads
  • Source
    • "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. "
    [Show abstract] [Hide abstract]
    ABSTRACT: Neurologic and carcinogenic effects caused by prolonged exposure to high linear energy transfer (LET) ionizing radiation (IR) represent major health-limiting obstacles during an estimated two-three year mission to Mars. Examination of DNA damage caused by high or low LET exposures has shown that high LET IR exposures cause far greater formation of multiply damaged sites, including complex DNA double strand breaks (DSBs) that are more difficult for cells to repair correctly. Presumably, these lesions lead to neurologic (cognitive and visual), and visual (cataracts) defects, as well as increased carcinogenic effects. This original research article focuses on potential health risks for individuals with defects in factors that function to accurately terminate RNA transcription, a process involving specific key steps in transcription, including: (i) accurate mRNA, miRNA, siRNA or lncRNA maturation, (ii) RNA polymerase II (RNA Pol II) stalling and dislodging; and (iii) subsequent resolution of DNA:RNA:DNA hybrids, known as 'R-loops'. Defects in RNA Pol II processing, in general, lead to persistent R-loop formation and ultimately formation of complex DSBs caused by unstable RNA:DNA structures and collisions between persistent R-loops and DNA replication and/or RNA transcriptional processes. We offer original data as evidence that low LET IR exposures cause delayed formation of persistent R-loops in wild-type cells. Further, we show that these unique and understudied DNA lesions result in indirect complex DSBs, breaks not generated by original deposition of energy. We also show that cells deficient in one RNA termination factor, Kub5-Hera (K-H/RPRD1B) show major defective DSB repair kinetics after high LET IR treatments, consistent with far greater R-loop formation and delayed and slower repair of persistent R-loop-derived DSBs than after low LET IR doses. Thus, haplo-insufficient loss of one essential RNA transcription termination scaffold factor, K-H (aka., RPRD1B), results in simultaneous defective repair of both R-loops and complex DSBs created by low or high LET IR. Whole body low LET IR-exposed haplo-insufficient K-H +/-mice showed hypersensitive carcinogenesis in a dose-dependent manner. These mice also have neurological and visual defects, including loss of hind limb function and
  • Source
    • "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. "
    [Show abstract] [Hide abstract]
    ABSTRACT: RecA/Rad51 catalyzed pairing of homologous DNA strands, initiated by polymerization of the recombinase on single-stranded DNA (ssDNA), is a universal feature of homologous recombination (HR). Generation of ssDNA from a double-strand break (DSB) requires nucleolytic degradation of the 5'-terminated strands to generate 3'-ssDNA tails, a process referred to as 5'-3' end resection. The RecBCD helicase-nuclease complex is the main end-processing machine in Gram-negative bacteria. Mre11-Rad50 and Mre11-Rad50-Xrs2/Nbs1 can play a direct role in end resection in archaea and eukaryota, respectively, by removing end-blocking lesions and act indirectly by recruiting the helicases and nucleases responsible for extensive resection. In eukaryotic cells, the initiation of end resection has emerged as a critical regulatory step to differentiate between homology-dependent and end-joining repair of DSBs.
    Cold Spring Harbor perspectives in biology 08/2014; 6(8). DOI:10.1101/cshperspect.a016436 · 8.68 Impact Factor
  • Source
    • "EXO1 has been proven to be directly involved in DNA repair mechanism[18], [27], [28]. Thus, we next explored whether EXO1 accounts for FOXM1-mediated cisplatin resistance. "
    [Show abstract] [Hide abstract]
    ABSTRACT: Cisplatin is commonly used in ovarian cancer chemotherapy, however, chemoresistance to cisplatin remains a great clinical challenge. Oncogenic transcriptional factor FOXM1 has been reported to be overexpressed in ovarian cancer. In this study, we aimed to investigate the potential role of FOXM1 in ovarian cancers with chemoresistance to cisplatin. Our results indicate that FOXM1 is upregulated in chemoresistant ovarian cancer samples, and defends ovarian cancer cells against cytotoxicity of cisplatin. FOXM1 facilitates DNA repair through regulating direct transcriptional target EXO1 to protect ovarian cancer cells from cisplatin-mediated apoptosis. Attenuating FOXM1 and EXO1 expression by small interfering RNA, augments the chemotherapy efficacy against ovarian cancer. Our findings indicate that targeting FOXM1 and its target gene EXO1 could improve cisplatin effect in ovarian cancer, confirming their role in modulating cisplatin sensitivity.
    PLoS ONE 05/2014; 9(5):e96989. DOI:10.1371/journal.pone.0096989 · 3.23 Impact Factor
Show more


56 Reads
Available from

Emma Bolderson