194 | Appendix English summary The adaptive immune system is renowned for its ability to elicit specific responses against pathogens, mediated by lymphocytes and antigen-presenting cells. B cells produce antibodies and utilize B cell receptors (BCR), also known as antigen receptors or immunoglobulins, to detect and eliminate pathogens, damaged, or non-self cells, leading to humoral immunity. T cells, on the other hand, employ T cell receptors (TCR) to eliminate foreign antigens, resulting in cellular immunity. The generation of a wide array of antigen receptors occurs through a sophisticated system called V(D)J recombination. Specifically found in precursor B and T cells, this process is initiated by the recombination activating gene 1 and 2 (RAG1/2) complex, which induces DNA breaks at recombination signal sequences (RSS), specific sequences abundant in immunoglobulin (Ig) and Tcr gene segments. These gene segments are brought together through a DNA recombination process in various combinations. RAG1/2 recognizes specific recombination signal sequences (RSS), flanking the coding V(D)J segments, and introduces a break in one of the DNA strands, called a single-stranded break. Subsequently, the 3’ hydroxyl end of the broken DNA strand induces a break in the opposite DNA strand, ultimately forming the double-strand DNA break (DSB). These types of DNA breaks are recognized by the MRE11/RAD50/NBS1 (MRN) protein complex, which binds near DNA breaks and recruits other DNA repair-associated proteins such as DNA-dependent protein kinase, catalytic subunit (DNA-PKcs), ataxia telangiectasia mutated (ATM), and Artemis. Cells use a process called non-homologous end joining (NHEJ) to repair double-strand DNA breaks. This means that no homology is required (no sister chromatid or homologous chromosome) to repair the break. Compared to other DNA repair processes, such as homologous recombination (HR), NHEJ is a much less accurate process: nucleotides, that were not originally in the germ line configuration, are incorporated into the junction, resulting in a shift in the genetic reading frame. This form of genetic “sloppiness” is advantageous for precursor B cells. On one hand, by introducing new nucleotides/mutations into the recombinant V(D)J gene segments, new, unique Ig receptors with unique specificities are constantly generated. However, on the other hand, this process can also introduce genetic errors elsewhere in the genome that may contribute to the malignant transformation of precursor B cells. Considering that DNA breaks are introduced during V(D)J gene recombination, this process must be tightly regulated to prevent DNA breaks from being introduced into non-Ig gene segments in the genome. How is RAG1/2-mediated DNA cutting kept within the boundaries of Ig genes? Is the regulation indeed so strict, or can RAG1/2 occasionally introduce DNA breaks in genes unrelated to Ig rearrangement, so-called “off-targets”? If this is the case, what if these breaks are also repaired with errors? Is there a link between the off-target RAG1/2 activity and the occurrence of genetic lesions that underlie the for-
RkJQdWJsaXNoZXIy MTk4NDMw