DNA polymerase ζ generates tandem mutations in immunoglobulin variable regions.

Low-fidelity DNA polymerases introduce nucleotide substitutions in immunoglobulin variable regions during somatic hypermutation. Although DNA polymerase (pol) eta is the major low-fidelity polymerase, other DNA polymerases may also contribute. Existing data are contradictory as to whether pol zeta is involved. We reasoned that the presence of pol eta may mask the contribution of pol zeta, and therefore we generated mice deficient for pol eta and heterozygous for pol zeta. The frequency and spectra of hypermutation was unaltered between Pol zeta(+/-) Pol eta(-/-) and Pol zeta(+/+) Pol eta(-/-) clones. However, there was a decrease in tandem double-base substitutions in Pol zeta(+/-) Pol eta(-/-). cells compared with Pol zeta(+/+) Pol eta(-/-) cells, suggesting that pol zeta generates tandem mutations. Contiguous mutations are consistent with the biochemical property of pol zeta to extend a mismatch with a second mutation. The presence of this unique signature implies that pol zeta contributes to mutational synthesis in vivo. Additionally, data on tandem mutations from wild type, Pol zeta(+/-), Pol zeta(-/-), Ung(-/-), Msh2(-/-), Msh6(-/-), and Ung(-/-) Msh2(-/-) clones suggest that pol zeta may function in the MSH2-MSH6 pathway.