MutSalpha binds to and promotes synapsis of transcriptionally activated immunoglobulin switch regions.

Immunoglobulin class switch recombination joins a new constant (C) region to the rearranged and expressed heavy chain variable (VDJ) region in antigen-activated B cells (Figure 1A) (reviewed in [1, 2]). Switch recombination is activated by transcription of intronic, G-rich and repetitive switch (S) regions and produces junctions that are heterogeneous in sequence and position in the S regions. Switch recombination depends upon the B cell-specific cytidine deaminase, AID, and conserved DNA repair factors, including the mismatch repair heterodimer, MutSalpha (MSH2/MSH6). In mice, ablation of Msh2 or Msh6, but not Msh3, decreases levels of switch recombination and diminishes heterogeneity of switch junctions [3-7]. Here, we demonstrate that MSH2 associates with transcribed S regions in primary murine B cells activated for switch recombination. Electron microscopic imaging reveals that MutSalpha binds in vitro to DNA structures formed within transcribed S regions and mediates their synapsis. MutSalpha binds with high affinity to G4 DNA formed upon transcription of the S regions and also binds to U.G mismatches, initial products of DNA deamination by AID. These results suggest that MutSalpha interacts with the S regions in switching B cells to promote DNA synapsis and recombination.