Microhomology-mediated end joining: new players join the team
© The Author(s) 2017
Received: 25 December 2016
Accepted: 6 January 2017
Published: 13 January 2017
DNA double-strand breaks (DSBs) are the most deleterious type of DNA damage in cells arising from endogenous and exogenous attacks on the genomic DNA. Timely and properly repair of DSBs is important for genomic integrity and survival. MMEJ is an error-prone repair mechanism for DSBs, which relies on exposed microhomologous sequence flanking broken junction to fix DSBs in a Ku- and ligase IV-independent manner. Recently, significant progress has been made in MMEJ mechanism study. In this review, we will summarize its biochemical activities of several newly identified MMEJ factors and their biological significance.
KeywordsDNA double-strand breaks (DSBs) Microhomology-mediated end joining (MMEJ) End resection RPA Polθ
Double-strand breaks (DSBs) are potentially lethal lesions that arise from endogenous and exogenous genotoxic agents [1, 2]. Unrepaired DSBs cause chromosome breaks and translocations that are associated with developmental defects, neurodegeneration, immunodeficiency, radiosensitivity, sterility, and cancer predisposition [3–5]. Non-homologous end joining (NHEJ) and homologous recombination (HR)-mediated DSB repair are two major pathways to fix DSBs [6, 7]. HR is generally considered to be an error-free mechanism because the identical sister chromatids are used as templates to repair DSBs when cells reside at the S and G2 phases. Ku-dependent classical non-homologous end joining (C-NHEJ) is active in all phases of the cell cycle, which can be high fidelity or associate with small alterations at junction since direct end ligation is catalyzed by DNA ligase IV [8–10]. In the absence of Ku protein or in C-NHEJ-deficient cells, resection machinery will expose extensive single strand DNA (ssDNA) which allows cells to use alternative end join (A-NHEJ) or HR as repair mechanism. A subset of A-NHEJ relies on microhomologous sequences on either side of the DSB, thus is named as microhomology-mediated end joining (MMEJ) [10–12]. MMEJ is a mutagenic DSB repair mechanism, which always associates with deletions flanking the break sites and contributes to chromosome translocations and rearrangements. Recent study indicated that MMEJ is used with appreciable frequency even when HR is available . It seems that MMEJ is a crucial DSB repair mechanism for HR-defective tumors . These raised the possibility that MMEJ may not just is a back-up repair mechanism. The molecular mechanism of MMEJ thus draws much attention in the field. Several important MMEJ factors have been identified recently [14–17]. Here, we will discuss biochemical properties and regulatory mechanism of these pivotal factors in MMEJ repair.
Basic mechanisms of MMEJ
Resection factors: mechanisms are still missing
In principle, both HR and MMEJ are initiated by 5′–3′ resection of DSB ends to expose ssDNA overhangs. While HR needs a long 3′-ssDNA tail to invade homologous template, MMEJ requires exposure of two microhomologous regions to anneal each other. Studies in yeast and mammalian cells indicated that DSB end resection may be carried out in two steps: Mre11 complex and Sae2/CtIP remove covalent adducts, such as bound proteins and hairpin-capped ends and initiate end resection. Sgs1/Exo1 and DNA2 in yeast or BLM (human homologue of Sgs1) and Exo1 in human cells take over to produce extended 3′-ssDNA tail [23–28]. It has been demonstrated that both Mre11 and CtIP are important for MMEJ. However, depletion of long-range resection factors including BLM/Exo1 in mammalian cells and Sgs1/Exo1 in yeast significantly increased frequency of MMEJ when the microhomologous regions close to the break site [13, 16, 29]. Possibly, down-regulation of long-range end resection may cause accumulation of short 3’tail containing DSBs which cannot be channeled to HR repair but is sufficient for exposing microhomologous region nearby DSB site and mediating MMEJ. However, we cannot rule out other possibilities yet. For example, some resection factors may harbor multiple functions. Further, the contradictory results have been obtained in studies of BRCA1, which also is a classical DSB end resection factor. BRCA1 closely associates with MRN complex and CtIP. CDK phosphorylation-mediated interaction between CtIP and BRCA1 enhances the speed of CtIP-mediated end resection . Cell cycle dependent BRCA1-MRN-CtIP complex formation has been reported to play a critical role in DSB end resection and HR-mediated DSB repair in mammalian cells . Early work in DT40 (chicken) B cells suggested that MMEJ is not affected by BRCA1 . While, using different human cells, a recent study indicated BRCA1 may work downstream of Mre11 and CtIP to suppress MMEJ . However, in MEFs cells whose telomeres were artificially uncapped, Madalena Tarsounas’s group demonstrated that CtIP and BRCA1 promote MMEJ at uncapped telomeres . Obviously, more accurate systems are needed to clarify the underlining mechanism for the functional relationship between resection factors and MMEJ.
RPA: an old soldier joined new team
Polθ: new focus
Increasing evidences suggest that MMEJ may not just be a back-up DSB repair mechanism. MMEJ occurs even when HR and NHEJ are intact and is essential for HR-deficient cancer cells. Therefore, it is well deserved to fully decipher the molecular mechanisms of MMEJ and its unique function in DSB repair. So far, several key factors identified in both MMEJ repair and regulation have overlapping functions with other repair pathways. Discovery of specific enzymes or protein factors that solely work in MMEJ repair pathway will help us understand the detail mechanism of MMEJ and its unique role in DSB repair and be instrumental for MMEJ-targeted drug design.
HW and XX planned and critically revised the manuscript. Both authors read and approved the final manuscript.
We thank the members of Xu and Wang lab for helpful discussions. We apologize for that we were not able to cite all the works of our colleagues in this review due to space limitation.
The authors declare that they have no competing interests.
Research in the Hailong Wang’s group and Xingzhi Xu’s group is supported by the 973 projects 2015CB910601/2 and 2013CB911002; the National Natural Science Foundation of China (NSFC) Grants 31370841, 31530016, and 31461143012; The Importation and Development of High-Caliber Talents Project of Beijing Municipal Institutions (CIT&TCD201504069).
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