Activation induced deaminase C-terminal domain links DNA breaks to end protection and repair during class switch recombination

Abstract

Activation-induced deaminase (AID) triggers antibody class switch recombination (CSR) in B cells by initiating DNA double strand breaks that are repaired by nonhomologous end-joining pathways. A role for AID at the repair step is unclear. We show that specific inactivation of the C-terminal AID domain encoded by exon 5 (E5) allows very efficient deamination of the AID target regions but greatly impacts the efficiency and quality of subsequent DNA repair. Specifically eliminating E5 not only precludes CSR but also, causes an atypical, enzymatic activity-dependent dominant-negative effect on CSR. Moreover, the E5 domain is required for the formation of AID-dependent Igh-cMyc chromosomal translocations. DNA breaks at the Igh switch regions induced by AID lacking E5 display defective end joining, failing to recruit DNA damage response factors and undergoing extensive end resection. These defects lead to nonproductive resolutions, such as rearrangements and homologous recombination that can antagonize CSR. Our results can explain the autosomal dominant inheritance of AID variants with truncated E5 in patients with hyper-IgM syndrome 2 and establish that AID, through the E5 domain, provides a link between DNA damage and repair during CSR.antibody gene diversification | isotype switching | somatic hypermutation

Decreased somatic hypermutation induces an impaired peripheral B cell tolerance checkpoint

“…These data are also in agreement with the identification of increased microhomology sequences in switched regions of AID +/-memory B cells, a characteristic associated with less efficient CSR through decreased AID activity (56). Dominant AICDA mutations encoding hyperactive AID variants that cannot catalyze CSR also resulted in decreased SHM frequencies in IgM + memory/marginal zone B cells (57). This paradoxical observation associating hyperactive AID enzyme activity with lower SHM may be reconciled with the possibility that hyperactive AID activity may induce exacerbated DNA lesions in GC B cells, causing B cells that attempt many rounds of antibody gene mutations to undergo apoptosis and be eliminated.…”

Functional requirements of AID’s higher order structures and their interaction with RNA-binding proteins

“…Because the AID C-terminal mutants were all shown to be monomeric, their wide distribution along the gradient suggested that some of the monomers associated with other proteins and RNAs. Collectively, these results showed that AID distributed in the LMW region could be subdivided into a narrow region of exclusive monomer (fractions 3-4) and a broader region of monomer and dimer (fractions [5][6][7][8][9][10][11][12] and that the HMW fractions of AID seemed to contain dimers.…”

“…In the present study, we demonstrated that specific AID-associated RBPs have the potential to direct AID's function toward either DNA cleavage or recombination. Because CSR recombination can be regulated at either the S-S synapse or the DNA end-processing repair phase (6,11,12), we speculate that at least two independent mRNAs are involved (7,11,60). Thus, it is reasonable to propose that distinct RBPs are involved in AID-RNP complex formation and that their KD affects different stages of CSR.…”

“…Mutations at the N terminus of AID impair SHM as well as CSR whereas those at the C terminus abrogate CSR only and show increased SHM activity. Recent studies demonstrated that the CSR process after DNA cleavage, including the synapsis formation between cleaved ends, is impaired with the C-terminally defective AID, indicating that AID's C terminus confers a CSR-specific recombination function, independent of AID's DNA cleavage function, by an unknown mechanism (11,12).AID belongs to the APOBEC (apolipoprotein B mRNA-editing enzyme catalytic polypeptide) family of cytidine deaminases (CDDs) and shows high sequence homology with APOBEC1 (A1) (1,13,14), which edits apolipoprotein B (APOB) mRNA. The APOB mRNA editing ability of A1 is highly dependent on its cofactors, A1CF/ACF (15, 16) and RBM47 (17), both of which belong to the heterogeneous nuclear ribonucleoprotein (hnRNP) family.…”

“…Mutations at the N terminus of AID impair SHM as well as CSR whereas those at the C terminus abrogate CSR only and show increased SHM activity. Recent studies demonstrated that the CSR process after DNA cleavage, including the synapsis formation between cleaved ends, is impaired with the C-terminally defective AID, indicating that AID's C terminus confers a CSR-specific recombination function, independent of AID's DNA cleavage function, by an unknown mechanism (11,12).…”