Introduction Double strand break DSB is the most deleterious
Introduction Double strand break (DSB) is the most deleterious damage that threats DNA integrity. This frequent damage arises from environmental stress like UV, chemical agents or ionizing radiations (IRs). Although environmental stress does not lead directly to DSB, it produces glycerol phosphate damage, mutations and DNA abnormalities. When they are not repaired, lesions cause single strand gaps (SSGs) or distortion in the DNA helix and this mechanical stress eventually leads to DSB. Endogenous stress is also a major cause of DNA damage. Exposure to endogenous reactive oxygen species may provoke DSBs. DSBs may also arise from polymerase operating errors or at stalled replication fork . In developing lymphocytes, DSB is required to initiate V(D)J recombination and Class Switch Recombination (CSR), two essential processes for the development of T and B cells, and Immunoglobulins (Igs) production , . However, the consequences of frequent DSB can be dramatic on genome stability. It can lead to DNA and RNA polymerases stalling, uncompleted replication, and chromosomal cross-over and as a general rule to the loss of genetic information. During evolution, several DNA damage repair pathways have been selected to face the diversity of DNA damages. These repair pathways take part to the global DNA Damage Response (DDR) of cells. The majority of mutations and single base damages due to exogenous stress (UV, chemicals..) or replication error are detected by specific mechanisms, i.e. base excision repair (BER), nucleotide excision repair (NER) and mismatch repair (MM). The first step of these pathways is cleavage and excision of DNA in order to replace the altered DNA. If uncompleted, the gap blocks replication and eventually causes DSBs . DSB repair is essential to the cell survival and DNA integrity. Three different DSB repair pathways exist; Homologous Recombination (HR) and Non-Homologous End Junction (NHEJ) are predominant while the microhomology-mediated end joining (MMEJ), also named alternative NHEJ (Alt-NHEJ) is considered as secondary (Fig. 1). HR is initiated by ends resection, which allows single strand invasion in a homologous region and a D-Loop formation. In humans, the Mre11-Rad50-NSB1 (MRN) complex associated with CtBP-interacting protein (CtIP) initiates 5′ nick to create single strand extremities. Subsequently, RAD51 promotes strand invasion in a homologous region, and new DNA is copied from the complementary strand (Fig. 1B). HR is predominant during S and G2 phase. Indeed, proximity of the sister chromatid allows an accessible homologous template, and promote HR. This pathway is considered as error-free . On the contrary, NHEJ promotes the ligation of two DNA blunt ends without DNA synthesis. Fast binding of the Ku70/Ku80 heterodimer (Ku) at breaks stabilizes DNA ends and allows recruitment of the DNA-dependent Protein Kinase catalytic subunit (DNA-PKcs) and ligation of the two ends (Fig. 1A). This pathway is faster and completed within 30 min compared to several hours for HR . However, when multiple breaks occur, NHEJ may induce improper joining between DNA ends and drive chromosomal translocations, depletions or duplications. This error-prone pathway is predominant in humans during G0 and G1 phase but is active in all phases. It is indeed considered that the fast replication of the whole DNA is preferred for transmission. On the other hand, since non-coding regions make up an extensive part of DNA in eukaryotes, there is statistically less chance for errors to occur in a coding region . A third alternative pathway, MMEJ or alt-NHEJ has been recently unveiled. Although this mechanism of repair is considered as highly error-prone, studies showed an important role in DSB repair depending on the biological context. MMEJ promotes ligation by microhomology-mediated annealing of the two ends. The CtIP/MRN complex is recruited by poly(ADP-ribose) polymerase 1 (PARP1) at the DNA breaks, which promote single-strand nick, and extensive resection of ends by the Bloom syndrome RecQ-like helicase (BLM)/exonuclease 1 (EXO1) to expose flanking microhomologies. It results in two single stranded ends flanking the DSB that anneal together by microhomology of 2 to 20 overlapping bases. XPF/ERCC1 nuclease trims non-homologous tails followed by the extension of the strand by the low fidelity DNA polymerase θ prior ligation by ligase 3 or ligase 1  (Fig. 1C).