Two publications from Cejka’s laboratory describe how cells repair broken DNA
on Tuesday, February 26, 2019
DNA bears all genetic information required for proper development and function of all living organisms. DNA molecule is however prone to breakage, threatening cell survival. Incorrect DNA break repair leads to mutations, which may drive tumorigenesis. In order to repair broken DNA without errors, cell utilize genetic information stored in the sister chromatid as a backup that provides instructions for repair. Importantly, the sister chromatids are only present in dividing cells upon completion of DNA replication, i.e. during the S and G2 phases of the cell cycle.
The first paper, entitled: "NBS1 promotes the endonuclease of the MRE11-RAD50 complex by sensing CtIP phosphorylation" was published in the EMBO Journal. Dr. Roopesh Anand and colleagues from the Institute for Research in Biomedicine in Bellinzona, as well as collaborators from the University of Zurich lead by Dr. Manuel Stucki, describe how the initiation of this process is regulated. They find how phosphorylation of CtIP, sensed by the NBS1 protein, controls the MRE11-RAD50 nuclease. Once properly activated, the molecular ensemble starts the first step in the recombination-based DNA break repair, termed DNA end resection. This regulatory control mechanism makes sure that recombination is not started until it is appropriate otherwise it would lead to DNA translocation and mutagenesis.
A model for MRN and CtIP functions in endonucleolytic DNA cleavage : when phosphorylated CtIP and MRN are present (in S-G2 phases), CtIP phosphorylation is detected by the FHA and BRCT domains of NBS1 (1), which in turn promotes DNA cleavage by MR via direct interaction with MRE11, mediated by the MRE11 interaction region of NBS1 (2). This results in maximal DNA end resection activity compatible with homologous recombination.
NBS1 promotes the endonuclease activity of the MRE11-RAD50 complex by sensing CtIP phosphorylation
R. Anand, A. Jasrotia, D. Bundschuh, S. M. Howard, L. Ranjha, M. Stucki, P. Cejka
in EMBO J (2019); DOI: 10.15252/embj.2018101005
The second paper was published in the Proceedings of the National Academy of Sciences (PNAS) and is entitled "Stepwise 5' DNA end specific resection of DNA breaks by the Mre11-Rad50-Xrs2 and Sae2 nuclease ensemble". Here, Dr. Elda Cannavo, Giordano Reginato and Dr. Petr Cejka from the Institute for Research in Biomedicine in Bellinzona identify how the DNA end resection machinery preferentially degrades the 5'-terminated DNA strand at DNA break sites. This polarity of DNA end resection is required to produce a 3'-overhang that is essential for the downstream recombinational repair. Cejka and colleagues here identify a mechanism that protects the 3' end and targets the nuclease complex to the 5'-terminated strand.
Models for short-range DNA end resection by MRX-pSae2. (A) Data presented in this work support a model in which the MRX complex, in a reaction stimulated by pSae2, degrades the 5’-terminated DNA strand by stepwise endonucleolytic incisions. In this model, one MRX-pSae2 complex promotes cleavage by another complex that binds DNA at an adjacent site. The endonucleolytic cleavage is followed by exonucleolytic degradation of the DNA fragments between the incision sites in a 3′→5’ direction. Degradation of the 5’ strand protects the 3’ end from 3’ →5’ exonuclease of MRX. (B) Data from yeast meiotic cells suggest that the first endonucleotic DNA cleavage occurs further away from the end (28). (C) A model that is a combination of A and B. One MRX-pSae2 complex may direct 5’ strand cleavage by another complex that binds DNA further away from the end.
Defects in human MRE11, CtIP and NBS1 proteins result in a variety of rare genetic disorders, including ataxia-telangiectasia-like disorder (ATLD), Seckel and Jawad syndrome, as well as Nijmegen breakage syndrome. Furthermore, DNA degradation by MRE11 was found important for the efficacy of tumor therapy by PARP inhibitors, which is applied for a subset of tumors deficient in BRCA1/2-proteins. Understanding the molecular mechanisms that regulate the MRE11 nuclease is thus, hoped to provide directions for the improvement of current therapeutic strategies.