Class switch recombination and DNA repair

Antibody class switch recombination (CSR) occurs in activated B cells and results in a switch from the production of IgM to IgG, IgA or IgE antibodies (Abs). CSR occurs by a DNA recombination event that joins double-strand breaks in switch (S) region sequences located upstream of Ig heavy chain constant gene segments. Previous studies have shown that efficient CSR requires the mismatch repair (MMR) proteins, MSH2, MLH1, and Exonuclease 1 (EXO1). Studies presented here provide evidence for the functional role of MMR proteins during CSR. Analyses of switch junction structures and CSR efficiency in the absence of both the mu switch region tandem repeats (SmuTR) and either MLH1 or EXO1 revealed a collective role of MMR proteins in CSR events. Specifically, these results show deficiency of MLH1 or EXO1 almost completely eliminates CSR in the absence of the SmuTR. Furthermore, in SmuTR-/- mice, deficiencies of MLH1 or EXO1 result in increased switch junction microhomology, similar to results obtained in the absence of MSH2. These results suggest the MMR machinery is important in processing DNA nicks into double-stranded DNA breaks, particularly in sequences where nicks are infrequent.;Additional studies presented here show that the MLH1, MSH2 and EXO1 proteins are important for CSR events in the absence of the classical-nonhomologous end-joining (NHEJ) pathway. Results show deficiency of the core NHEJ factor, XRCC4, and either MLH1, EXO1, or MSH2 reduces CSR efficiency more than single protein loss. Additionally, removal of MMR proteins from XRCC4-deficient B cells results in fewer CSR induced IgH locus translocations and non-ligatable DNA breaks.;These studies support a model that describes the major function of MMR proteins in CSR as converting DNA point mutations and single-strand nicks into double-strand DNA breaks used by multiple DNA end joining mechanisms for CSR. We also find that switch junction structures are altered in Mlh1-XRCC4-mice and chromosome translocations are slightly decreased in Exo1-XRCC4-mice compared to XRCC4-mice alone. These results suggest MMR proteins may have additional functions independent of other MMR proteins that influence the alternative-end joining (A-EJ) pathway of CSR.