DNA polymerase θ contributes to the generation of C/G mutations during somatic hypermutation of Ig genes

Somatic hypermutation of Ig variable region genes is initiated by activation-induced cyticline deaminase; however, the activity of multiple DNA polymerases is required to ultimately introduce mutations. DNA polymerase eta (Pol eta) has been implicated in mutations at A/T, but polymerases involved in C/G mutations have not been identified. We have generated mutant mice expressing DNA polymerase (Polo) specifically devoid of polymerase activity. Compared with WT mice, Polq-inactive (Polq, the gene encoding Pol theta) mice exhibited a reduced level of serum lgM and IgG1. The mutant mice mounted relatively normal primary and secondary immune responses to a T-dependent antigen, but the production of high-affinity specific antibodies was partially impaired. Analysis of the J(H)4 intronic sequences revealed a slight reduction in the overall mutation frequency in Polq-inactive mice. Remarkably, although mutations at A/T were unaffected, mutations at C/G were significantly decreased, indicating an important, albeit not exclusive, role for Polo activity. The reduction of C/G mutations was particularly focused on the intrinsic somatic hypermutation hotspots and both transitions and transversions were similarly reduced. These findings, together with the recent observation that Polo efficiently catalyzes the bypass of abasic sites, lead us to propose that Pol theta introduces mutations at C/G by replicating over abasic sites generated via uracil-DNA glycosylase.