The DNA glycosylases Ogg1 and Nth1 do not contribute to Ig class switching in activated mouse splenic B cells

DNA glycosylase deficiency leads to decreased severity of lupus in the Polb-Y265C mouse model

The Polb gene encodes DNA polymerase beta (Pol β), a DNA polymerase that functions in base excision repair (BER) and microhomology-mediated end-joining. The Pol β-Y265C protein exhibits low catalytic activity and fidelity, and is also deficient in microhomology-mediated end-joining. We have previously shown that the Polb^Y265C/+ and Polb^Y265C/C mice develop lupus. These mice exhibit high levels of antinuclear antibodies and severe glomerulonephritis. We also demonstrated that the low catalytic activity of the Pol β-Y265C protein resulted in accumulation of BER intermediates that lead to cell death. Debris released from dying cells in our mice could drive development of lupus. We hypothesized that deletion of the Neil1 and Ogg1 DNA glycosylases that act upstream of Pol β during BER would result in accumulation of fewer BER intermediates, resulting in less severe lupus. We found that high levels of antinuclear antibodies are present in the sera of Polb^Y265C/+ mice deleted of Ogg1 and Neil1 DNA glycosylases. However, these mice develop significantly less severe renal disease, most likely due to high levels of IgM in their sera.

Base Excision Repair in the Immune System: Small DNA Lesions With Big Consequences

The integrity of the genome is under constant threat of environmental and endogenous agents that cause DNA damage. Endogenous damage is particularly pervasive, occurring at an estimated rate of 10,000–30,000 per cell/per day, and mostly involves chemical DNA base lesions caused by oxidation, depurination, alkylation, and deamination. The base excision repair (BER) pathway is primary responsible for removing and repairing these small base lesions that would otherwise lead to mutations or DNA breaks during replication. Next to preventing DNA mutations and damage, the BER pathway is also involved in mutagenic processes in B cells during immunoglobulin (Ig) class switch recombination (CSR) and somatic hypermutation (SHM), which are instigated by uracil (U) lesions derived from activation-induced cytidine deaminase (AID) activity. BER is required for the processing of AID-induced lesions into DNA double strand breaks (DSB) that are required for CSR, and is of pivotal importance for determining the mutagenic outcome of uracil lesions during SHM. Although uracils are generally efficiently repaired by error-free BER, this process is surprisingly error-prone at the Ig loci in proliferating B cells. Breakdown of this high-fidelity process outside of the Ig loci has been linked to mutations observed in B-cell tumors and DNA breaks and chromosomal translocations in activated B cells. Next to its role in preventing cancer, BER has also been implicated in immune tolerance. Several defects in BER components have been associated with autoimmune diseases, and animal models have shown that BER defects can cause autoimmunity in a B-cell intrinsic and extrinsic fashion. In this review we discuss the contribution of BER to genomic integrity in the context of immune receptor diversification, cancer and autoimmune diseases.

Role of DNA repair in host immune response and inflammation

In recent years, the understanding of how DNA repair contributes to the development of innate and acquired immunity has emerged. The DNA damage incurred during the inflammatory response triggers the activation of DNA repair pathways, which are required for host-cell survival. Here, we reviewed current understanding of the mechanism by which DNA repair contributes to protection against the oxidized DNA damage generated during infectious and inflammatory diseases and its involvement in innate and adaptive immunity. We discussed the functional role of DNA repair enzymes in the immune activation and the relevance of these processes to: transcriptional regulation of cytokines and other genes involved in the inflammatory response; V(D)J recombination; class-switch recombination (CSR); and somatic hypermutation (SHM). These three last processes of DNA damage repair are required for effective humoral adaptive immunity, creating genetic diversity in developing T and B cells. Furthermore, viral replication is also dependent on host DNA repair mechanisms. Therefore, the elucidation of the pathways of DNA damage and its repair that activate innate and adaptive immunity will be important for a better understanding of the immune and inflammatory disorders and developing new therapeutic interventions for treatment of these diseases and for improving their outcome.

Mismatch repair proteins and AID activity are required for the dominant negative function of C-terminally deleted AID in class switching

Activation-induced cytidine deaminase (AID) is essential for class-switch recombination (CSR) and somatic hypermutation (SHM) of Ig genes. The AID C terminus is required for CSR, but not for S-region DNA double-strand breaks (DSBs) during CSR, and it is not required for SHM. AID lacking the C terminus (ΔAID) is a dominant negative (DN) mutant, because human patients heterozygous for this mutant fail to undergo CSR. In agreement, we show that ΔAID is a DN mutant when expressed in AID-sufficient mouse splenic B cells. To have DN function, ΔAID must have deaminase activity, suggesting that its ability to induce DSBs is important for the DN function. Supporting this hypothesis, Msh2-Msh6 have been shown to contribute to DSB formation in S regions, and we find in this study that Msh2 is required for the DN activity, because ΔAID is not a DN mutant in msh2(-/-) cells. Our results suggest that the DNA DSBs induced by ΔAID are unable to participate in CSR and might interfere with the ability of full-length AID to participate in CSR. We propose that ΔAID is impaired in its ability to recruit nonhomologous end joining repair factors, resulting in accumulation of DSBs that undergo aberrant resection. Supporting this hypothesis, we find that the S-S junctions induced by ΔAID have longer microhomologies than do those induced by full-length AID. In addition, our data suggest that AID binds Sμ regions in vivo as a monomer.