DNA polymerase β prevents AID-instigated mutagenic non-canonical mismatch DNA repair

Abstract

In B cells, the error-prone repair of activation-induced cytidine deaminase (AID)-induced lesions in immunoglobulin variable genes cause somatic hypermutation (SHM) of antibody genes. Due to clonal selection in the germinal centers (GC) this active mutation process provides the molecular basis for antibody affinity maturation. AID deaminates cytosine (C) to create uracil (U) in DNA. Typically, the short patch base excision repair (spBER) effectively restores genomic U lesions. We here demonstrate that GC B cells actively degrade DNA polymerase β (Polβ), resulting in the inactivation of the gap-filling step of spBER. Consequently, lesions instigated by AID, and likely other base damages, are channeled towards mutagenic non-canonical mismatch repair (mncMMR), responsible for the vast majority of mutations at adenine and thymine (A:T) bases. Apparently, GC B cells prohibit faithful spBER, thereby favoring A:T mutagenesis during SHM. Lastly, our data suggest that the loss of Polβ relates to hypoxia that characterizes the GC microenvironment.

Huwe1 supports B-cell development, B-cell-dependent immunity, somatic hypermutation and class switch recombination by regulating proliferation

“…Given these insights, it is apparent that both SHM and CSR involve the complex interplay between the BER and the MMR pathways, where the proximal part of the BER pathway (UNG and APE1/2) contributes to mutagenesis during SHM and to DNA nicks during CSR, whereas the distal BER component POLB counteracts mutagenesis and CSR. In agreement, we have recently shown that loss of POLB stimulates in vitro A/T mutagenesis by triggering EXO1 patch excision and ncMMR activity ( 10 ). Furthermore, we showed that human GC B cells are characterized by low protein expression of POLB, whereas B-cell lines that are used to study SHM and CSR in vitro display ample POLB protein expression, which may explain the relative paucity of A/T mutations in these cell lines.…”

“…As a result, the clonotypic class switched antibody of a B-cell is equipped with an alternate Ch with distinct effector functions and tissue distribution. Interestingly, the generation of DNA nicks required for CSR may be counteracted by conventional BER involving POLB, which faithfully repairs AID-instigated U lesions ( 9 , 10 ).…”

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

“…This in part explains the lack of a GC phenotype in mice with POLB deficient lymphocytes. Furthermore, our studies suggest that POLB protein is destabilized in GC B cells due to the hypoxic microenvironment that marks the GC (18,249,250). Our data suggests that the specific loss of POLB in GC B cells is instrumental in the mutagenic repair of AID-dependent lesions (Figure 2).…”

“…AgR diversification mechanisms are associated with hallmark genetic aberrations in B-cell lymphomas, mostly derived from illegitimate CSR events and off-target mutations caused by AID (16,17). Mutagenic repair is coupled to a partial inactivation of the short-patch BER pathway in rapidly dividing GC B cells (Figure 2), which contributes to genomic instability (18) (preprint). Accordingly, the majority of human B-cell lymphomas are derived from the GC (19).…”

Tandem Substitutions in Somatic Hypermutation

“…Indeed, this is in line with our previous observation that tandem substitutions are more abundant in immunoglobulin loci than the genome at large, as UNG activity is mainly directed to these genetic regions. Additionally, relative absence of faithful DNA repair mechanisms during SHM makes UNG-derived AP sites more vulnerable to tandem substitutions than genome-wide events of spontaneous deamination (59).…”