Different mutation signatures in DNA polymerase η- and MSH6-deficient mice suggest separate roles in antibody diversification

Hypermutation in immunoglobulin genes produces a high frequency of substitutions of all four bases, which are likely generated by low-fidelity DNA polymerases. Indeed, humans deficient for DNA polymerase (pol) η have decreased substitutions of A·T base pairs in variable and switch regions. To study the role of pol η in a genetically tractable system, we created mice lacking pol η. B cells from Polh^-/- mice produced normal amounts of IgG, indicating that pol η does not affect class switch recombination. Similar to their human counterparts, variable and switch regions from Polh^-/- mice had fewer substitutions of A·T base pairs and correspondingly more mutations of C·G base pairs, which firmly establishes a central role for pol η in hypermutation. Notably, the location and types of substitutions differ markedly from those in Msh6^-/- clones, which also have fewer A·T mutations. The data suggest that pol η preferentially synthesizes a repair patch on the nontranscribed strand, whereas MSH6 functions to generate the patch.