Correlation of somatic hypermutation specificity and A-T base pair substitution errors by DNA polymerase eta during copying of a mouse immunoglobulin kappa light chain transgene

To test the hypothesis that inaccurate DNA synthesis by mammalian DNA polymerase η (pol η) contributes to somatic hypermutation (SHM) of Ig genes, we measured the error specificity of mouse pol η during synthesis of each strand of a mouse Ig κ light chain transgene. We then compared the results to the base substitution specificity of SHM of this same gene in the mouse. Thein vitroandin vivobase substitution spectra shared a number of common features. A highly significant correlation was observed for overall substitutions at A-T pairs but not for substitutions at G-C pairs. Sixteen mutational hotspots at A-T pairs observedin vivowere also found in spectra generated by mouse pol ηin vitro. The correlation was strongest for errors made by pol η during synthesis of the non-transcribed strand, but it was also observed for synthesis of the transcribed strand. These facts, and the distribution of substitutions generatedin vivo, support the hypothesis that pol η contributes to SHM of Ig genes at A-T pairs via short patches of low fidelity DNA synthesis of both strands, but with a preference for the non-transcribed strand.