Article

CtIP promotes microhomology-mediated alternative end joining during class-switch recombination

Immunology Program, Memorial Sloan-Kettering Cancer Center, New York, New York, USA.
Nature Structural & Molecular Biology (Impact Factor: 13.31). 01/2011; 18(1):75-9. DOI: 10.1038/nsmb.1942
Source: PubMed

ABSTRACT Immunoglobulin heavy chain (Igh locus) class-switch recombination (CSR) requires targeted introduction of DNA double strand breaks (DSBs) into repetitive 'switch'-region DNA elements in the Igh locus and subsequent ligation between distal DSBs. Both canonical nonhomologous end joining (C-NHEJ) that seals DNA ends with little or no homology and a poorly defined alternative end joining (A-NHEJ, also known as alt-NHEJ) process that requires microhomology ends for ligation have been implicated in CSR. Here, we show that the DNA end-processing factor CtIP is required for microhomology-directed A-NHEJ during CSR. Additionally, we demonstrate that microhomology joins that are enriched upon depletion of the C-NHEJ component Ku70 require CtIP. Finally, we show that CtIP binds to switch-region DNA in a fashion dependent on activation-induced cytidine deaminase. Our results establish CtIP as a bona fide component of microhomology-dependent A-NHEJ and unmask a hitherto unrecognized physiological role of microhomology-mediated end joining in a C-NHEJ-proficient environment.

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    • "It processes ssDNA gaps of 2 to 25 nucleotides in length (McVey & Lee, 2008), while ssDNA gaps 430 nucleotides are repaired by ssDNA annealing (SSA). Proteins involved in MMEJ include BLM/MRN, EXO1 or DNA2, FEN1, DNA polymerase b, or m, Ligase I or Ligase III/XRCC1 and MMR proteins (Figure 1) (Crespan et al., 2012; Lee-Theilen et al., 2011; Nimonkar et al., 2011; Paul et al., 2013). After ssDNA is annealed in the gap region, mismatched bases are corrected by MMR. "
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    • "Several enzymatic activities have been implicated in this repair pathway, which is now considered to be distinct from D-NHEJ and which may even be further subdivided into sub-pathways (Wang et al., 2005; Rosidi et al., 2008; Zha et al., 2009; Lee-Theilen et al., 2011; Mladenov and Iliakis, 2011). A major protein implicated in B-NHEJ is poly (ADP-ribose) polymerase 1 (PARP-1), which plays a main role in the repair of SSBs (see below) and which may effectively compete with KU heterodimer for DNA end-binding (Wang et al., 2006). "
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    • "see Mladenov and Iliakis, 2011); classical NHEJ depending on the activities of at least 7 identified factors (i.e. Ku70, Ku80, DNA-PKcs, Arthemis, XRCC4, Cernunos (also called XRCC4-like factor, XLF) and Ligase IV) and alternative NHEJ which depends on MRN trimmer complex but its repair activity is, obviously, independent of proteins needed for classical pathway (Corneo et al., 2007, Yan et al., 2007; Deriano et al., 2009; Lee-Theilen et al., 2011). This alternative NHEJ was demonstrated to be error-prone and consequently, mutagenic since it uses microhomology pairing and thus nucleotides loss. "
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