Dual Functions of Nbs1 in the Repair of DNA Breaks and Proliferation Ensure Proper V(D)J Recombination and T-Cell Development

Abstract

DNA double-strand breaks (DSBs) are generated by exogenous DNA damage stimuli (e.g., ionizing radiation) and endogenous metabolic intermediates, such as collapsed replication forks. Upon DSB generation, NBS1 together with MRE11 and RAD50 form the MRN complex (26). MRE11, which contains two DNA binding domains, mediates the MRN complex binding to the exposed DNA ends. The adjacent MRNassociated DNA ends could be further tethered together through the coiled-coil domain (zinc hook) of RAD50 (19,43). The chromatin loading of MRN stimulates the ATM kinase that subsequently phosphorylates the downstream effectors, such as p53 and CHK2, for cell cycle regulation, apoptosis, and repair initiation (9,27,40). In addition to transducing the repair signal, the NBS1/MRN complex participates in modulating DNA damage response (DDR) pathways by promoting error-free homology-directed repair (HDR) while repressing nonhomologous end joining (NHEJ) to minimize the generation of errant, therefore potentially "dangerous," DNA joints in S-and G 2 /M-phase cells (47).Mutations in any of the MRN complex proteins increase genomic instability and cause human genomic instability disorders. NBS1 hypomorphism causes the autosomal recessive disorder Nijmegen breakage syndrome (NBS; OMIM 251260).NBS patients show multisystemic defects, among which immunodeficiency and a predisposition to lymphoid malignancies originating from B-or T-cell lineages are the major hallmarks (11,22). NBS patients are prone to opportunistic infections due to agammaglobulinemia, primarily IgA and IgG. Other immune defects include a reduced total number of CD3 ϩ and CD4 ϩ but not CD8 ϩ single-positive (SP) T cells (11,22,30). The MRN complex is therefore proposed to play an important role in the sensing (as a DDR component) and repairing (as a component of NHEJ) of V(D)J-generated DSBs and thereby to have its regulatory function in lymphoid development (2, 13). However, NBS patient cells carrying hypomorphic mutations of the NBS1 gene fail to demonstrate a direct role of NBS1 in V(D)J recombination (15,20,48).During lymphocyte development, G 1 -phase-specific endonucleases RAG1/2 generate the DSBs at recombination signal sequences (RSS) flanking functional V, D, and J gene segments, which are in turn joined mainly by the NHEJ machinery (13, 28). Recent studies using intrachromosomal recombination substrates in the MRN hypomorphic mouse B cells or a small interfering RNA (siRNA) knockdown of MRN in cell lines demonstrate that the MRN complex stabilizes the DSB ends generated by RAG1/2 to promote canonical NHEJ (C-NHEJ; LigIV-Xrcc4 dependent) and also alternative NHEJ (A-NHEJ; LigIV-Xrcc4 independent) if C-NHEJ is not available (8,10,12,17,32,34,44,46). NBS1 consists of multiple domains and interacts with several key DDR molecules at both the early or late stages of the DDR cascade (49). While hypomorphic mutation supports cell and mouse survival, the complete loss of any of the MRN components causes embryonic lethality, suggesting a vital role of Nbs1

Interaction between NBS1 and the mTOR/Rictor/SIN1 Complex through Specific Domains

“…This result is the further extension of the ability of NBS1 to interact with both the upstream PI3K and downstream Akt to regulate the activity of PI3K/Akt pathway. It is also supported by recent literatures demonstrating the dual function of NBS1 in the repair of DNA breaks and proliferation and the requirement of NBS1 in IGF-1 induced cellular proliferation [25], [26]. In addition, the ability of RNA regulon to promote Akt activity also goes through the upregulation of NBS1 levels [27].…”

Programmed DNA breaks in lymphoid cells: repair mechanisms and consequences in human disease

“…In murine models, loss of NBS1 hinders T‐cell development at an early developmental stage 60. Mechanistically, NBS1 depletion compromises loading of the MRN complex to V(D)J‐generated DSBs and thereby affects DNA end resection 60…”

Defining ATM-Independent Functions of the Mre11 Complex with a Novel Mouse Model

“…Most of the studies concerning the role of the Mre11 complex in these processes rely on effects observed upon deletion of one of its subunits (6) (7). However, it is conceivable that abnormal cell cycle progression, proliferation and increased mortality consequent to Mre11 complex ablation have impeded an appropriate characterization of its influence, as well as its relationship with ATM, in the repair of physiological breaks.…”