Shifts in targeting of class switch recombination sites in mice that lack mu 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 Smu tandem repeats show shifts indicating that a 4-5-kb DNA domain (bounded upstream by the Imu promoter) is accessible for switching independent of Smu 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 Smu 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.