Genome Plasticity and B cell
This group belongs to the group "TLS Polymerases: Genome Plasticity and Cancers"
Our group associates researchers working on the coordination and regulation of DNA repair processes in B lymphocyte following AID expression and on the role of AID in B cell tumourigenesis.
To create the necessary diversity and specificity, the immune system uses two successive and regulated genomic alterations. Early in B cell development, the antigen receptors (B cell receptor or antibody expressed at B cell surface) are constructed by V(D)J recombination, a site-specific process initiated by the lymphocyte specific proteins RAG1 and RAG2. The mechanism by which V(D)J recombination occurs is clearly established, involving DNA double-strand break intermediaries which are resolved by the ubiquitous non-homologous end-joining DNA repair machinery. However, in response to antigens, mammalian B cells undergo a new series of specialised DNA modifications resulting in the production of specific antibodies with both high affinity and the adequate effector function. These modifications are somatic hypermutations (SHM), in which single base pair changes are preferentially introduced in the variable domain of immunoglobulin (Ig) with a very high frequency, and class switch recombination (CSR), in which recombination between switch sequences leads to a change of antibody isotype. These processes depend on activation-induced cytidine deaminase (AID), a key enzyme expressed only in B cells from secondary lymphoid organs upon antigen encounter. AID is a cytidine deaminase acting directly on DNA. To generate SHM starting from U:G mismatches, AID participates in concert with the other partners of the “mutasome” including mismatch repair proteins (Msh2, Msh6 and Exo1), mono-ubiquitinated PCNA and the translesion synthesis error prone polymerases pol eta and rev1. Immunoglobulin class switch recombination is also initiated by AID. U:G mismatches and abasic sites (following the dU removal by uracil DNA glycosylase (Ung2)) are processed to produce lesions that recruit and activate DNA damage response proteins including Ataxia-telangiectasia mutated (ATM), histone H2AX, Nijmegen breakage syndrome 1 (Nbs1), and p53 binding protein 1 (53BP1).
In general the presence of uracil on DNA is recognised and efficiently removed and replaced by the correct base by the base excision repair machinery. Surprisingly, in activated B cells during physiological Ig gene SHM, the uracils generated by AID seem to be actively ignored and used to promote more mutations, in particular at A-T residues, which account for half of the mutations generated in vivo. AID activity therefore seems to be able not only to generate dU residues at the Ig loci, but also to activate error-prone repair pathways that convert it to mutation.
The group’s project aims to address and to study how the B cell machinery coordinates two very challenging processes: First, the obligation to shape and adapt quickly and efficiently the receptor to a given antigen (to increase the affinity and specificity) by introducing a high rate of mutations in the Ig variable domain and by initiating DNA breaks for class switch recombination. Second, providing the necessary protection of the rest of the genome from both deleterious mutations due to aberrant SHM and translocations associated with aberrant CSR. Although many of the enzymes that are involved have been identified in the past few years, some of the mutational factors have yet to be discovered, and there is still much to be learned about how they are coordinated and targeted to the Variable and Switch regions. Finally, the group, in collaboration with medical doctors from Gustave Roussy haematology department will address the role of the “mutasome” machinery in the onset of human germinal centres B cell non-Hodgkin's lymphomas.
