| Abstract | Excision of uracil introduced into the immunoglobulin loci by AID is central to antibody diversification. While predominantly carried out by the UNG uracil-DNA glycosylase as reflected by deficiency in immunoglobulin class switching in Ung-/- mice, the deficiency is incomplete, as evidenced by the emergence of switched IgG in the serum of Ung-/- mice. Lack of switching in mice deficient in both UNG and MSH2 suggested that mismatch repair initiated a backup pathway. We now show that most of the residual class switching in Ung-/- mice depends upon the endogenous SMUG1 uracil-DNA glycosylase, with in vitro switching to IgG1 as well as serum IgG3, IgG2b, and IgA greatly diminished in Ung-/-Smug1-/- mice, and that Smug1 partially compensates for Ung deficiency over time. Nonetheless, using a highly MSH2-dependent mechanism, Ung-/-Smug1-/- mice can still produce detectable levels of switched isotypes, especially IgG1. While not affecting the pattern of base substitutions, SMUG1 deficiency in an Ung-/- background further reduces somatic hypermutation at A:T base pairs. Our data reveal an essential requirement for uracil excision in class switching and in facilitating noncanonical mismatch repair for the A:T phase of hypermutation presumably by creating nicks near the U:G lesion recognized by MSH2. |
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