Durandy, A. | bioRxiv Science

bioRxiv2019

Activation-induced deaminase is critical for the establishment of DNA methylation patterns prior to the germinal center reaction

Mutations in activation induced deaminase (AID) lead to hyper-IgM syndrome type 2 (HIGM2), a rare human primary antibody deficiency. AID-mediated cytosine deamination has been proposed as mediating active demethylation, although evidences both support and cast doubt on such a role. We here made use of HIGM2 B cells to investigate direct AID involvement in active DNA demethylation. HIGM2 naive and memory B cells both display widespread DNA methylation defects, of which approximately 25% of these defects correspond to active events. For genes that undergo active demethylation that is impaired in HIGM2 individuals, we did not observe AID involvement but a participation of TET enzymes. DNA methylation alterations in HIGM2 naive B cells are related to premature overstimulation of the B-cell receptor prior to the germinal center reaction. Our data supports a role for AID in B cell central tolerance in preventing the expansion of autoreactive cell clones, affecting the correct establishment of DNA methylation patterns.

immunology↗

bioRxiv2018

Locus Suicide Recombination actively occurs on the functionally rearranged IgH allele in B-cells from inflamed human lymphoid tissues

B-cell activation yields abundant cell death in parallel to clonal amplification and remodeling of immunoglobulin (Ig) genes by activation-induced deaminase (AID). AID promotes affinity maturation of Ig variable regions and class switch recombination (CSR) in mature B lymphocytes. In the IgH locus, these processes are under control by the 3 regulatory region (3RR) super-enhancer, a region demonstrated in the mouse to be both transcribed and itself targeted by AID-mediated recombination. Alternatively to CSR, IgH deletions joining S to \"like-switch\" DNA repeats that flank the 3 super-enhancer can thus accomplish so-called \"locus suicide recombination\" (LSR) in mouse B-cells. We now show that AID-mediated LSR also actively occurs in humans, and provides an activation-induced cell death pathway in multiple conditions of B-cell activation. LSR deletions either focus on the functional IgH allele or are bi-allelic, since they can only be detected when they are ongoing and their signature vanishes from fully differentiated plasma cells or from \"resting\" blood memory B-cells, but readily reappears when such memory B-cells are re-stimulated in vitro. Highly diversified breakpoints are distributed either within the upstream (3RR1) or downstream (3RR2) copies of the IgH 3 super-enhancer and all conditions activating CSR in vitro also seem to trigger LSR.\n\nAuthor SummaryClass switch recombination, initiated by the activation-induced deaminase enzyme rearranges immunoglobulin (Ig) genes in order to replace expression of IgM by IgG, IgA or IgE. A variant form of this event, locus suicide recombination (LSR), was previously reported in mouse B-lymphocytes and simply deletes all functional Ig constant genes, thus terminating B-cell function. This study first demonstrates that the structure of the human Ig heavy chain locus provides an ideal target for LSR, and is thus actively (but transiently) affected by this deletional process at the activated B-cell stage. LSR then yields recombined genes that do not support B-cell survival and which thus become undetectable among long-lived memory B-cells or plasma cells.

genetics↗