title: Activation-induced cytidine deaminase-dependent DNA breaks in class switch recombination occur during G1 phase of the cell cycle and depend upon mismatch repair.
abstract: Ab class switching occurs by an intrachromosomal recombination and requires generation of double-strand breaks (DSBs) in Ig switch (S) regions. Activation-induced cytidine deaminase (AID) converts cytosines in S regions to uracils, which are excised by uracil DNA glycosylase (UNG). Repair of the resulting abasic sites would yield single-strand breaks (SSBs), but how these SSBs are converted to DSBs is unclear. In mouse splenic B cells, we find that AID-dependent DSBs occur in Smu mainly in the G(1) phase of the cell cycle, indicating they are not created by replication across SSBs. Also, G(1) phase cells express AID, UNG, and mismatch repair (MMR) proteins and possess UNG activity. We find fewer S region DSBs in MMR-deficient B cells than in wild-type B cells, and still fewer in MMR-deficient/SmuTR(-/-) B cells, where targets for AID are sparse. These DSBs occur predominantly at AID targets. We also show that nucleotide excision repair does not contribute to class switching. Our data support the hypothesis that MMR is required to convert SSBs into DSBs when SSBs on opposite strands are too distal to form DSBs spontaneously.
authors.affiliation: Department of Molecular Genetics and Microbiology, Program in Immunology and Virology, University of Massachusetts Medical School, Worcester, MA 01655, USA. Carol.Schrader@umassmed.edu
authors.forename: Carol E
authors.initials: CE
authors.lastname: Schrader
authors.forename: Jeroen E J
authors.initials: JE
authors.lastname: Guikema
authors.forename: Erin K
authors.initials: EK
authors.lastname: Linehan
authors.forename: Erik
authors.initials: E
authors.lastname: Selsing
authors.forename: Janet
authors.initials: J
authors.lastname: Stavnezer
mesh_terms: D000818:Animals; D001483:Base Sequence; D002478:Cells, Cultured; D003564:Cytidine Deaminase / Q000172:deficiency / Q000235:genetics / Q000378:metabolism*; D004247:DNA / Q000235:genetics*; D004249:DNA Damage / Q000235:genetics*; D053843:DNA Mismatch Repair; D016193:G1 Phase / Q000235:genetics* / Q000276:immunology; D017578:Immunoglobulin Class Switching / Q000235:genetics* / Q000276:immunology*; D051379:Mice; D018345:Mice, Knockout; D008969:Molecular Sequence Data; D051981:Uracil-DNA Glycosidase / Q000378:metabolism; D000098830:AICDA (Activation-Induced Cytidine Deaminase)
chemical_list: D004247:DNA; D051981:Uracil-DNA Glycosidase; D000098830:AICDA (Activation-Induced Cytidine Deaminase); D003564:Cytidine Deaminase