PCNA ubiquitination-independent activation of polymerase η during somatic hypermutation and DNA damage tolerance

The generation of high affinity antibodies in B cells critically depends on translesion synthesis (TLS) polymerases that introduce mutations into immunoglobulin genes during somatic hypermutation (SHM). The majority of mutations at A/T base pairs during SHM require ubiquitination of PCNA at lysine 164 (PCNA-Ub), which activates TLS polymerases. By comparing the mutation spectra in B cells of WT, TLS polymerase eta (Pol eta)-deficient, PCNA(K164R)-mutant, and PCNA(K164R);Pol eta double-mutant mice, we now find that most PCNA-Ub-independent A/T mutagenesis during SHM is mediated by Pol eta. In addition, upon exposure to various DNA damaging agents, PCNA(K164R) mutant cells display strongly impaired recruitment of TLS polymerases, reduced daughter strand maturation and hypersensitivity. Interestingly, compared to the single mutants, PCNA(K164R);Pol eta double-mutant cells are dramatically delayed in S phase progression and far more prone to cell death following UV exposure. Taken together, these data support the existence of PCNA ubiquitination-dependent and -independent activation pathways of Pol eta during SHM and DNA damage tolerance. (C) 2011 Elsevier B.V. All rights reserved.