Uracil excision by endogenous SMUG1 glycosylase promotes efficient Ig class switching and impacts on A:T substitutions during somatic mutation.
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.
MRC Laboratory of Molecular Biology, Cambridge, UK.
Felix A
FA
Dingler
Kristin
K
Kemmerich
Michael S
MS
Neuberger
Cristina
C
Rada
D000818:Animals; D007070:Immunoglobulin A / Q000097:blood; D017578:Immunoglobulin Class Switching; D007074:Immunoglobulin G / Q000097:blood; D051379:Mice; D051718:MutS Homolog 2 Protein / Q000502:physiology; D009154:Mutation; D014498:Uracil / Q000378:metabolism*; D051981:Uracil-DNA Glycosidase / Q000502:physiology*
D007070:Immunoglobulin A; D007074:Immunoglobulin G; D014498:Uracil; D051981:Uracil-DNA Glycosidase; C497173:Msh2 protein, mouse; D051718:MutS Homolog 2 Protein
Class switching; DNA deamination; Somatic hypermutation; Uracil