The structure-selective endonucleases GEN1 and MUS81 mediate complementary functions in safeguarding the genome of proliferating B lymphocytes.

During the development of humoral immunity, activated B lymphocytes undergo vigorous proliferative, transcriptional, metabolic, and DNA remodeling activities; hence, their genomes are constantly exposed to an onslaught of genotoxic agents and processes. Branched DNA intermediates generated during replication and recombinational repair pose genomic threats if left unresolved and so, they must be eliminated by structure-selective endonucleases to preserve the integrity of these DNA transactions for the faithful duplication and propagation of genetic information. To investigate the role of two such enzymes, GEN1 and MUS81, in B cell biology, we established B-cell conditional knockout mouse models and found that deletion of GEN1 and MUS81 in early B-cell precursors abrogates the development and maturation of B-lineage cells while the loss of these enzymes in mature B cells inhibit the generation of robust germinal centers.

The uncharacterized SANT and BTB domain-containing protein SANBR inhibits class switch recombination.

Class switch recombination (CSR) is the process by which B cells switch production from IgM/IgD to other immunoglobulin isotypes, enabling them to mount an effective immune response against pathogens. Timely resolution of CSR prevents damage due to an uncontrolled and prolonged immune response. While many positive regulators of CSR have been described, negative regulators of CSR are relatively unknown.

AID Phosphorylation Regulates Mismatch Repair-Dependent Class Switch Recombination and Affinity Maturation.

Activation-induced cytidine deaminase (AID) generates U:G mismatches in Ig genes that can be converted into untemplated mutations during somatic hypermutation or DNA double-strand breaks during class switch recombination (CSR). Null mutations in and demonstrate the complementary roles of the base excision repair (BER) and mismatch repair pathways, respectively, in CSR. Phosphorylation of AID at serine 38 was previously hypothesized to regulate BER during CSR, as the AID phosphorylation mutant, AID(S38A), cannot interact with APE1, a BER protein.

Binding of AID to DNA does not correlate with mutator activity.

The DNA deaminase activation-induced cytidine deaminase (AID) initiates somatic hypermutation (SHM) and class switch recombination (CSR) by deaminating cytidines to uridines at V region (V) genes and switch (S) regions. The mechanism by which AID is recruited to V genes and S region DNA is poorly understood. In this study, we used the CH12 B lymphoma line to demonstrate that, although S regions can efficiently recruit AID and undergo mutations and deletions, AID neither binds to nor mutates the V gene, thus clearly demonstrating intraimmunoglobulin locus specificity.

Regulation of immunoglobulin class-switch recombination: choreography of noncoding transcription, targeted DNA deamination, and long-range DNA repair.

Upon encountering antigens, mature IgM-positive B lymphocytes undergo class-switch recombination (CSR) wherein exons encoding the default Cμ constant coding gene segment of the immunoglobulin (Ig) heavy-chain (Igh) locus are excised and replaced with a new constant gene segment (referred to as "Ch genes", e.g., Cγ, Cɛ, or Cα).

The splicing regulator PTBP2 interacts with the cytidine deaminase AID and promotes binding of AID to switch-region DNA.

During immunoglobulin class-switch recombination (CSR), the cytidine deaminase AID induces double-strand breaks into transcribed, repetitive DNA elements called switch sequences. The mechanism that promotes the binding of AID specifically to switch regions remains to be elucidated. Here we used a proteomic screen with in vivo biotinylation of AID to identify the splicing regulator PTBP2 as a protein that interacts with AID.

CtIP promotes microhomology-mediated alternative end joining during class-switch recombination.

Immunoglobulin heavy chain (Igh locus) class-switch recombination (CSR) requires targeted introduction of DNA double strand breaks (DSBs) into repetitive 'switch'-region DNA elements in the Igh locus and subsequent ligation between distal DSBs. Both canonical nonhomologous end joining (C-NHEJ) that seals DNA ends with little or no homology and a poorly defined alternative end joining (A-NHEJ, also known as alt-NHEJ) process that requires microhomology ends for ligation have been implicated in CSR. Here, we show that the DNA end-processing factor CtIP is required for microhomology-directed A-NHEJ during CSR.

Viral double-stranded RNA triggers Ig class switching by activating upper respiratory mucosa B cells through an innate TLR3 pathway involving BAFF.

Class switch DNA recombination (CSR) from IgM to IgG and IgA is crucial for antiviral immunity. Follicular B cells undergo CSR upon engagement of CD40 by CD40 ligand on CD4+ T cells. This T cell-dependent pathway requires 5-7 days, which is too much of a delay to block quickly replicating pathogens.