A/T mutagenesis in hypermutated ARTICLE immunoglobulin genes strongly depends on PCNAK164 modification

B cells use translesion DNA synthesis (TLS) to introduce somatic mutations around genetic lesions caused by activation-induced cytidine deaminase. Monoubiquitination at lysine 164 of proliferating cell nuclear antigen (PCNA (K164R)) stimulates TLS. To determine the role of PCNA(K164) modifications in somatic hypermutation, PCNA(K164R) knock-in mice were generated. PCNA (K164R/K164R) mutants are born at a sub-Mendelian frequency. Although PCNA(K164R/K164R) B cells proliferate and class switch normally, the mutation spectrum of hypermutated immunoglobulin (19) genes alters dramatically. A strong reduction of mutations at template A/T is associated with a compensatory increase at G/C, which is a phenotype similar to polymerase eta (Pol eta) and mismatch repair-deficient B cells. Mismatch recognition, monoubiquitinated PCNA, and Pol eta likely cooperate in establishing mutations at template A/T during replication of 19 genes.