Department of Microbiology and Physiological Systems
Animals; B-Lymphocytes; DNA Glycosylases; *DNA Repair; DNA-(Apurinic or Apyrimidinic Site) Lyase; DNA-Binding Proteins; Endonucleases; Gene Expression Regulation, Enzymologic; Germinal Center; Mice; Mice, Knockout; MutS Homolog 2 Protein; *Mutation; Proliferating Cell Nuclear Antigen; Somatic Hypermutation, Immunoglobulin
Genetics and Genomics | Immunology and Infectious Disease
Somatic hypermutation (SHM) of antibody variable region genes is initiated in germinal center B cells during an immune response by activation-induced cytidine deaminase (AID), which converts cytosines to uracils. During accurate repair in nonmutating cells, uracil is excised by uracil DNA glycosylase (UNG), leaving abasic sites that are incised by AP endonuclease (APE) to create single-strand breaks, and the correct nucleotide is reinserted by DNA polymerase beta. During SHM, for unknown reasons, repair is error prone. There are two APE homologs in mammals and, surprisingly, APE1, in contrast to its high expression in both resting and in vitro-activated splenic B cells, is expressed at very low levels in mouse germinal center B cells where SHM occurs, and APE1 haploinsufficiency has very little effect on SHM. In contrast, the less efficient homolog, APE2, is highly expressed and contributes not only to the frequency of mutations, but also to the generation of mutations at A:T base pair (bp), insertions, and deletions. In the absence of both UNG and APE2, mutations at A:T bp are dramatically reduced. Single-strand breaks generated by APE2 could provide entry points for exonuclease recruited by the mismatch repair proteins Msh2-Msh6, and the known association of APE2 with proliferating cell nuclear antigen could recruit translesion polymerases to create mutations at AID-induced lesions and also at A:T bp. Our data provide new insight into error-prone repair of AID-induced lesions, which we propose is facilitated by down-regulation of APE1 and up-regulation of APE2 expression in germinal center B cells.
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Citation:Proc Natl Acad Sci U S A. 2014 Jun 24;111(25):9217-22. doi: 10.1073/pnas.1405590111. Epub 2014 Jun 9. Link to article on publisher's site
Proceedings of the National Academy of Sciences of the United States of America
Stavnezer, Janet; Linehan, Erin K.; Thompson, Mikayla R.; Habboub, Ghaith; Ucher, Anna J.; Kadungure, Tatenda; Tsuchimoto, Daisuke; Nakabeppu, Yusaku; and Schrader, Carol E., "Differential expression of APE1 and APE2 in germinal centers promotes error-prone repair and A:T mutations during somatic hypermutation" (2014). University of Massachusetts Medical School Faculty Publications. 694.