Shifts in targeting of class switch recombination sites in mice that lack μ switch region tandem repeats or Msh2

The mechanisms that target class switch recombination (CSR) to antibody gene switch (S) regions are unknown. Analyses of switch site locations in wild-type mice and in mice that lack the Sμ tandem repeats show shifts indicating that a 4–5-kb DNA domain (bounded upstream by the Iμ promoter) is accessible for switching independent of Sμ sequences. This CSR-accessible domain is reminiscent of the promoter-defined domains that target somatic hypermutation. Within the 4–5-kb CSR domain, the targeting of S site locations also depends on the Msh2 mismatch repair protein because Msh2-deficient mice show an increased focus of sites to the Sμ tandem repeat region. We propose that Msh2 affects S site location because sequences with few activation-induced cytidine deaminase targets generate mostly switch DNA cleavages that require Msh2-directed processing to allow CSR joining.