7 General discussion | 177 gen-activated protein kinase (MAPK), ERK or c-Jun N-terminal kinase (JNK)70,71. However, the inhibition of these kinases by small molecule inhibitors did not prevent the rapid DNA damage-induced downregulation of RAG. Allelic exclusion refers to a phenomenon in the immune system in which each lymphocyte expresses only one functional allele of its antigen receptor gene, ensuring specificity in immune responses. This process is crucial for generating a diverse repertoire of antigen receptors while preventing self-reactivity and autoimmune reactions. There are several proposed models explaining the mechanisms of allelic exclusion. The asynchronous recombination model, for instance, assumes that the probability of simultaneous bi-allelic recombinations is limited by the chromatin configurations, allowing recombination on one allele, while preventing recombination on the other allele. The feedback inhibition models suggest that the products or intermediates of Ig gene rearrangements act to inhibit the recombination process. According to the classical feedback inhibition model, successful Ig gene rearrangements induce signals via pre-BCRs or B-cell receptors (BCRs) to suppress additional allelic recombination72–74. In line with the current models, DSBs are thought to help enforce the allelic exclusion; the DNA breaks activate ATM, which in turn phosphorylates a plethora of downstream targets, including histone H2AX, modulating the accessibility of Ig loci for the RAG recombination machinery. Phosphorylated histone H2AX leads to the recruitment of chromatin remodeling complexes, such as the switch/ Sucrose non-fermenting (SWI/SNF) complex75. Based on the results of our study, we hypothesize that activation of ATM induces multiple negative feedback loops leading to transcriptional downregulation of RAG1/2 protein and mRNA, and thus enforcing allelic exclusion by limiting RAG’s availability for further recombinations. Previous studies have also demonstrated that ATM prevents secondary Igk recombinations by triggering a negative feedback loop, where the DSBs, generated during V to J recombination, eventually lead to inhibition of RAG1/2 recombinase76. Mechanistically, ATM-mediated repression of growth arrest and DNA-damage-inducible protein alpha (GADD45a) was suggested to contribute to this negative feedback loop69,76. However, in contrast to these previous studies, we did not detect any changes in protein or mRNA expression of GADD45a following NCS-induced DNA damage neither in human BCR-ABL1+ cell line nor in mouse v-Abl transformed cells. Consistent with our findings, the Bassing lab also demonstrated that in mouse pre-B cells the changes in GADD45a did not modulate the transcriptional expression of Rag1/2 in response to DNA damage77. Moreover, the Bassing lab also showed that in mouse pre-B cells, the inactivation of NF-κB essential modulator (Nemo), a known ATM signaling effector, resulted in increased Rag1 and Rag2 transcription, increased Igk accessibility, and triggered RAG-dependent DSBs. The inactivation of Nemo disrupted the repression of Rag1/2 mRNA and increased the simultaneous bi-allelic cleavage. Moreover, the induction of Spi-C transcription factor (SpiC) by RAG-mediated DSBs, led to a decrease
RkJQdWJsaXNoZXIy MTk4NDMw