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) eta have decreased substitutions of A-T base pairs in variable and switch regions. To study the role of pol eta in a genetically tractable system, we created mice lacking pol eta. B cells from Polh(-/-) mice produced normal amounts of IgG, indicating that pol eta 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 eta 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 eta preferentially synthesizes a repair patch on the nontranscribed strand, whereas MSH6 functions to generate the patch.