FAM72A antagonizes UNG2 to promote mutagenic uracil repair during antibody maturation

Abstract

Activation-induced cytidine deaminase (AID) catalyzes the deamination of deoxycytidines within Immunoglobulin (Ig) genes to induce somatic hypermutation (SHM) and class switch recombination (CSR). AID-induced deoxyuracils within Ig loci are recognized and processed by subverted base excision and mismatch repair pathways that ensure a mutagenic outcome in B lymphocytes. However, it is unclear why DNA repair pathways that remove deoxyuracil from DNA are not efficient at faithfully repairing AID-induced lesions. Here, we identified through a genome-wide CRISPR screen that FAM72A, a protein with no ascribed function, is a major determinant for the error-prone processing of deoxyuracil. Fam72a-deficient CH12F3-2 B cells and primary B cells from Fam72a-/- mice have reduced CSR and SHM frequencies. The SHM spectrum in B cells from Fam72a-/- mice is opposite to that observed in Ung2-/- mice, suggesting that UNG2 is hyperactive in FAM72A-deficient cells. Indeed, FAM72A binds to UNG2 resulting in reduced UNG2 activity, and significantly reduced protein levels in the G1 phase, coinciding with peak AID activity. This effect leads to increased genome-wide deoxyuracils in B cells. By antagonizing UNG2, FAM72A therefore increases U:G mispairs that engage mutagenic mismatch repair promoting error-prone processing of AID-induced deoxyuracils. This work shows that FAM72A bridges base-excision repair and mismatch repair to modulate antibody maturation.

Deep learning model of somatic hypermutation reveals importance of sequence context beyond targeting of AID and Polη hotspots

“…The trend for increased transition mutations at highly mutating AID hotspots mediated by UNG2 had previously been observed in experiments using 3T3 (mouse fibroblast) cells (Pérez-Durán et al, 2012), although the particular bias against C:G>A:T transversions was not apparent. Previous work has also shown that UNG2 is cell-cycle regulated, possibly mediated by FAM72A (Feng et al, 2020), and active primarily during G1 (Sharbeen et al, 2012). Although AID is also primarily active during G1, it may sometimes persist for slightly longer than UNG2 and thus highly targeted sites may avoid BER especially when the mutations occur just before the cell enters S phase, which would lead to fixation of C>T transitions via replication bypass.…”

Identification of a Chemotherapeutic Lead Molecule for the Potential Disruption of the FAM72A-UNG2 Interaction to Interfere with Genome Stability, Centromere Formation, and Genome Editing

“…Under pathophysiological conditions, FAM72A is also expressed in various proliferating cancer cells [ 24 , 25 ]. Notably, FAM72A interacts with UNG2 [ 26 , 27 ]. This denotes that the cellular role of FAM72A is as a cooperative partner for genomic BER in order to ensure genome integrity and impede the formation of cancer.…”

“…Recent data show that decreased levels of FAM72A lead to hyperphysiological UNG2 levels, an increased uracil correction, and, thus, error-free DNA repair. In contrast, the binding of FAM72A with UNG2 antagonizes UNG2 activity and causes UNG2 degradation in B cells, leading to increased levels of genome-wide deoxyuracils and, therefore, mediating increased levels of U•G mispairs that engage in mutagenic mismatch repair, promoting the error-prone processing of activation-induced cytidine deaminase (AID)-induced deoxyuracils [ 27 , 28 ]. Thus, FAM72A bridges BER and mismatch repair in order to modulate antibody diversification during B cell and antibody maturation [ 27 , 28 ].…”

“…In contrast, the binding of FAM72A with UNG2 antagonizes UNG2 activity and causes UNG2 degradation in B cells, leading to increased levels of genome-wide deoxyuracils and, therefore, mediating increased levels of U•G mispairs that engage in mutagenic mismatch repair, promoting the error-prone processing of activation-induced cytidine deaminase (AID)-induced deoxyuracils [ 27 , 28 ]. Thus, FAM72A bridges BER and mismatch repair in order to modulate antibody diversification during B cell and antibody maturation [ 27 , 28 ]. Overall, an increased FAM72A level could lead to reduced UNG2 levels and could thus shift the balance of appropriate mutagenic DNA repair, therefore making the cells more susceptible to mutations, with possible effects on tumor development [ 24 , 25 , 27 , 28 , 29 ].…”

“…Thus, FAM72A bridges BER and mismatch repair in order to modulate antibody diversification during B cell and antibody maturation [ 27 , 28 ]. Overall, an increased FAM72A level could lead to reduced UNG2 levels and could thus shift the balance of appropriate mutagenic DNA repair, therefore making the cells more susceptible to mutations, with possible effects on tumor development [ 24 , 25 , 27 , 28 , 29 ].…”