Ig gene somatic hypermutation in mice defective for DNA polymerase δ proofreading

This study is an investigation of the possible role of DNA polymerase (pol) δ with an inactivated exonuclease (exo) in somatic hypermutation (SHM). Analysis of endogenous heavy chain transcripts revealed no difference in mutation frequency and pattern between exo^-/-, exo^+/- and exo^+/+ mice. The lack of an effect of the pol δ exo mutation on SHM could be due to: (i) normally pol δ is used in SHM, but the exo is prevented from proofreading, (ii) normally pol δ is used, but the decrease in fidelity of the exo^- pol does not increase hypermutation frequency enough to be detected, and (iii) pol δ is not used in SHM. Based on the finding in the exo^-/- mice and the current understanding of the process of SHM, it is concluded that pol δ is not normally involved in creating the mutations. The majority of the mutated sequences obtained in this study, including many from the exo^-/- mice, were from genes which had switched to a γ heavy chain class. Thus, the pol δ proofreading activity is not required for class switch recombination (CSR). Genealogical trees observed with multiple mutated sequences of various Ig classes show that CSR and SHM occur intermingled during expansion of a cell clone, raising the possibility that they may occur at the same time.