AID-induced decrease in topoisomerase 1 induces DNA structural alteration and DNA cleavage for class switch recombination

Proc Natl Acad Sci U S A. 2009 Dec 29;106(52):22375-80. doi: 10.1073/pnas.0911879106. Epub 2009 Dec 11.


To initiate class switch recombination (CSR) activation-induced cytidine deaminase (AID) induces staggered nick cleavage in the S region, which lies 5' to each Ig constant region gene and is rich in palindromic sequences. Topoisomerase 1 (Top1) controls the supercoiling of DNA by nicking, rotating, and religating one strand of DNA. Curiously, Top1 reduction or AID overexpression causes the genomic instability. Here, we report that the inactivation of Top1 by its specific inhibitor camptothecin drastically blocked both the S region cleavage and CSR, indicating that Top1 is responsible for the S region cleavage in CSR. Surprisingly, AID expression suppressed Top1 mRNA translation and reduced its protein level. In addition, the decrease in the Top1 protein by RNA-mediated knockdown augmented the AID-dependent S region cleavage, as well as CSR. Furthermore, Top1 reduction altered DNA structure of the Smu region. Taken together, AID-induced Top1 reduction alters S region DNA structure probably to non-B form, on which Top1 can introduce nicks but cannot religate, resulting in S region cleavage.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • B-Lymphocytes / drug effects
  • B-Lymphocytes / immunology
  • B-Lymphocytes / metabolism
  • Camptothecin / pharmacology
  • Cell Line
  • Cytidine Deaminase / deficiency
  • Cytidine Deaminase / genetics
  • Cytidine Deaminase / metabolism*
  • DNA / chemistry*
  • DNA / genetics
  • DNA / metabolism*
  • DNA Topoisomerases, Type I / genetics
  • DNA Topoisomerases, Type I / metabolism*
  • Immunoglobulin Class Switching* / drug effects
  • In Vitro Techniques
  • Mice
  • Mice, Inbred C57BL
  • Mice, Knockout
  • Nucleic Acid Conformation
  • Protein Biosynthesis
  • RNA, Messenger / genetics
  • RNA, Messenger / metabolism
  • RNA, Small Interfering / genetics
  • Topoisomerase I Inhibitors


  • RNA, Messenger
  • RNA, Small Interfering
  • Topoisomerase I Inhibitors
  • DNA
  • AICDA (activation-induced cytidine deaminase)
  • Cytidine Deaminase
  • DNA Topoisomerases, Type I
  • Camptothecin