36 | Chapter 2 precursor from forming p52178. A B-cell specific TRAF2 and TRAF3 double deficiency in mice caused B cells to be more resistant to DNA damage-induced apoptosis, probably due to cellular inhibitor of apoptosis 2 (cIAP2) and X-linked inhibitor of apoptosis (XIAP), which in turn attenuates caspase-3 activation. However, in this model, apart from oligoclonal B-cell repertoire, no evidence was found that TRAF2/3 double deficiency would induce genomic instability in primary B cells179. Another more recent study generated mice that have either or both NF-kB pathways constitutively active as of the early pro-B cell stage, by crossing the TRAF3 conditional knock-out mice (Traf3flx/flx) and the IkB-kinase 2 (IKK2) transgenic mice (IKK2ca ) mice, which lead to a systemic loss of B cells180. In one of our studies, we demonstrated that in murine cycling pre-B cells, NF-kB signaling represses RAG expression through the inhibition of CDK4 and FOXO1 and thereby protects pre-B cells from RAG-induced genomic instability. Interestingly, we showed that AKT signaling at this stage feeds into the NF-kB/FOXO1 axis and potentiates its inhibitory effect on RAG expression. Furthermore, NF-kB and AKT also inhibit RAG expression in primary human pre-B acute lymphoblastic leukemia cells from patients. Therefore, targeting parts of this regulatory pathway with new therapies could accidentally increase the risk of malignant pre-B cell transformation by allowing uncontrolled RAG expression181. The regulatory function of RAG-induced DSBs Throughout their development, B cells have to repeatedly deal damage to their genetic material. Next to homeostatic sources of DNA damage, B cells sustain DNA damage as a result of V(D)J recombination during development and as a result of CSR and SHM upon activation and antigen encounter. Though DNA breaks can have devastating effects on genome integrity, they can also be exploited for precise regulation of various biological processes or decision-making activities. The DSBs trigger a conserved DNA damage response (DDR) in order to repair the genetic lesions, such a response is shared by most cell types. Sometimes, the activation of DDR kinases results in cell-type specific processes that do not have a known function in DNA damage repair, but instead seem to regulate other processes182. There are 2 central kinases in DDR: ATM kinase and ATM- and Rad3-related kinase (ATR). ATM is recruited to the sites containing DSBs upon MRN activation183. ATR primarily responds to a broad spectrum of DNA damage lesions, including single-stranded DNA. Both ATM and ATR are essential for checkpoint responses to the DSBs. Upon binding to the MRN complex, ATM activation takes place through autophosphorylation, which subsequently activates the heterodimer of KU70 (XRCC6) and KU80 (XRCC5) and other kinases involved in DNA repair184,185. The DSBs-induced phosphorylation of ATM at Ser1981 subsequently initiates phosphorylation of checkpoint kinase 2 (Chk2) at Thr68. Once activated, Chk2 phosphorylates more than 20 different proteins, mainly involved in cell cycle progression and apoptosis, such as p53,
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