Targeting of Somatic Hypermutation by Enhancer and Enhancer-Like Sequences

Somatic hypermutation (SH) generates point mutations within rearranged immunoglobulin (Ig) genes of activated B cells, providing genetic diversity for the affinity maturation of antibodies. SH requires the activation-induced cytidine deaminase (AID) protein and transcription of the mutation target sequence, but how the Ig gene specificity of mutations is achieved has remained elusive. We show here using a sensitive and carefully controlled assay that the Ig enhancers strongly activate SH in neighboring genes even though their stimulation of transcription is negligible. Mutations in certain E-box, NFkB, MEF2, or Ets family binding sites—known to be important for the transcriptional role of Ig enhancers—impair or abolish the activity. Full activation of SH typically requires a combination of multiple Ig enhancer and enhancer-like elements. The mechanism is evolutionarily conserved, as mammalian Ig lambda and Ig heavy chain intron enhancers efficiently stimulate hypermutation in chicken cells. Our results demonstrate a novel regulatory function for Ig enhancers, indicating that they either recruit AID or alter the accessibility of the nearby transcription units. Citation: Buerstedde J-M, Alinikula J, Arakawa H, McDonald JJ, Schatz DG (2014) Targeting of Somatic Hypermutation by mmunoglobulin Enhancer and Enhancer-Like Sequences. PLoS Biol 12(4): e1001831. doi:10.1371/journal.pbio.1001831 Academic Editor: David Nemazee, Scripps Research Institute, United States of America Received November 11, 2013; Accepted February 21, 2014; Published April 1, 2014 Copyright: 2014 Buerstedde et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: J-M.B. was supported by an International Outgoing Marie Curie Fellowship. J.J.M. was supported in part by NIH T32 AI07019. D.G.S is an investigator of the Howard Hughes Medical Institute. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. Abbreviations: GCV, immunoglobulin gene conversion; SH, somatic hypermutation. * E-mail: brstdd@gmail.com (J-MB); david.schatz@yale.edu (DGS)

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